[SMF] crash when malformed NAS message (open5gs#3132)

A malformed PDU Session Modification Request is sent from UE
after Registration Complete.

```
Crash 1:
04/12 15:00:44.031: [amf] INFO: [imsi-999700000000001:1:11][0:0:NULL] /nsmf-pdusession/v1/sm-contexts/{smContextRef}/modify (../src/amf/nsmf-handler.c:837)
04/12 15:00:46.569: [nas] FATAL: ogs_nas_parse_qos_flow_descriptions: Assertion `descriptions->length' failed. (../lib/nas/5gs/types.c:486)
04/12 15:00:46.569: [core] FATAL: backtrace() returned 11 addresses (../lib/core/ogs-abort.c:37)
../src/smf/../../lib/nas/5gs/libogsnas-5gs.so.2(ogs_nas_parse_qos_flow_descriptions+0x162) [0x7e6e7a5a4e5d]
../src/smf/open5gs-smfd(+0x8c6ec) [0x5dd6c333d6ec]
../src/smf/open5gs-smfd(+0x2d69b) [0x5dd6c32de69b]
../src/smf/../../lib/core/libogscore.so.2(ogs_fsm_dispatch+0x119) [0x7e6e7b216c0c]
../src/smf/open5gs-smfd(+0x288b3) [0x5dd6c32d98b3]
../src/smf/../../lib/core/libogscore.so.2(ogs_fsm_dispatch+0x119) [0x7e6e7b216c0c]
../src/smf/open5gs-smfd(+0xf2d8) [0x5dd6c32c02d8]
../src/smf/../../lib/core/libogscore.so.2(+0x1197a) [0x7e6e7b20797a]
/lib/x86_64-linux-gnu/libc.so.6(+0x94ac3) [0x7e6e7a094ac3]
/lib/x86_64-linux-gnu/libc.so.6(+0x126850) [0x7e6e7a126850]
04/12 15:00:46.613: [app] ERROR: Signal-NUM[17] received (Child status change) (../src/main.c:81)
04/12 15:00:46.613: [sbi] WARNING: [92] HTTP/2 stream 19 was not closed cleanly before end of the underlying stream (../lib/sbi/client.c:626)
04/12 15:00:46.613: [scp] WARNING: response_handler() failed [-1] (../src/scp/sbi-path.c:539)
04/12 15:00:46.613: [amf] ERROR: [1:0] No SmContextUpdateError [500] (../src/amf/nsmf-handler.c:866)
04/12 15:00:46.613: [amf] ERROR: AMF_SESS_CLEAR (../src/amf/amf-sm.c:484)
04/12 15:00:46.613: [amf] INFO: [Removed] Number of AMF-Sessions is now 0 (../src/amf/context.c:2551)
04/12 15:00:50.596: [nrf] WARNING: [c466ec64-f8fe-41ee-a888-194dc4363612] No heartbeat (../src/nrf/nrf-sm.c:260)
04/12 15:00:50.596: [nrf] INFO: [c466ec64-f8fe-41ee-a888-194dc4363612] NF de-registered (../src/nrf/nf-sm.c:205)
04/12 15:00:50.596: [sbi] INFO: [c466ec64-f8fe-41ee-a888-194dc4363612:1] NF removed (../lib/sbi/nnrf-handler.c:750)
04/12 15:00:50.596: [sbi] INFO: [c466ec64-f8fe-41ee-a888-194dc4363612:1] NF removed (../lib/sbi/nnrf-handler.c:750)
04/12 15:00:55.094: [pfcp] WARNING: [10] LOCAL  No Reponse. Give up! for step 1 type 1 peer [127.0.0.4]:8805 (../lib/pfcp/xact.c:599)
04/12 15:00:55.094: [upf] WARNING: No Heartbeat from SMF [127.0.0.4]:8805 (../src/upf/pfcp-sm.c:329)
04/12 15:00:55.094: [upf] INFO: PFCP de-associated [127.0.0.4]:8805 (../src/upf/pfcp-sm.c:199)
04/12 15:01:02.599: [pfcp] WARNING: [11] LOCAL  No Reponse. Give up! for step 1 type 5 peer [127.0.0.4]:8805 (../lib/pfcp/xact.c:599)
04/12 15:01:06.098: [upf] WARNING: Retry to association with peer [127.0.0.4]:8805 failed (../src/upf/pfcp-sm.c:107)

Crash 2:
04/12 15:16:39.748: [amf] INFO: [imsi-999700000000001:1:11][0:0:NULL] /nsmf-pdusession/v1/sm-contexts/{smContextRef}/modify (../src/amf/nsmf-handler.c:837)
04/12 15:16:42.155: [nas] FATAL: ogs_nas_parse_qos_rules: Assertion `size+sizeof(rule->flow.flags) <= length' failed. (../lib/nas/5gs/types.c:961)
04/12 15:16:42.155: [core] FATAL: backtrace() returned 11 addresses (../lib/core/ogs-abort.c:37)
../src/smf/../../lib/nas/5gs/libogsnas-5gs.so.2(ogs_nas_parse_qos_rules+0x12d1) [0x7d1affbd2d72]
../src/smf/open5gs-smfd(+0x8b446) [0x629a57861446]
../src/smf/open5gs-smfd(+0x2d69b) [0x629a5780369b]
../src/smf/../../lib/core/libogscore.so.2(ogs_fsm_dispatch+0x119) [0x7d1affd05c0c]
../src/smf/open5gs-smfd(+0x288b3) [0x629a577fe8b3]
../src/smf/../../lib/core/libogscore.so.2(ogs_fsm_dispatch+0x119) [0x7d1affd05c0c]
../src/smf/open5gs-smfd(+0xf2d8) [0x629a577e52d8]
../src/smf/../../lib/core/libogscore.so.2(+0x1197a) [0x7d1affcf697a]
/lib/x86_64-linux-gnu/libc.so.6(+0x94ac3) [0x7d1afea94ac3]
/lib/x86_64-linux-gnu/libc.so.6(+0x126850) [0x7d1afeb26850]
04/12 15:16:42.199: [sbi] WARNING: [92] HTTP/2 stream 13 was not closed cleanly before end of the underlying stream (../lib/sbi/client.c:626)
04/12 15:16:42.199: [scp] WARNING: response_handler() failed [-1] (../src/scp/sbi-path.c:539)
04/12 15:16:42.199: [app] ERROR: Signal-NUM[17] received (Child status change) (../src/main.c:81)
04/12 15:16:42.200: [amf] ERROR: [1:0] No SmContextUpdateError [500] (../src/amf/nsmf-handler.c:866)
04/12 15:16:42.200: [amf] ERROR: AMF_SESS_CLEAR (../src/amf/amf-sm.c:484)
04/12 15:16:42.200: [amf] INFO: [Removed] Number of AMF-Sessions is now 0 (../src/amf/context.c:2551)
04/12 15:16:49.858: [nrf] WARNING: [23f1aee2-f901-41ee-a488-85a58e1e3420] No heartbeat (../src/nrf/nrf-sm.c:260)
04/12 15:16:49.858: [nrf] INFO: [23f1aee2-f901-41ee-a488-85a58e1e3420] NF de-registered (../src/nrf/nf-sm.c:205)
04/12 15:16:49.859: [sbi] INFO: [23f1aee2-f901-41ee-a488-85a58e1e3420:1] NF removed (../lib/sbi/nnrf-handler.c:750)
04/12 15:16:49.859: [sbi] INFO: [23f1aee2-f901-41ee-a488-85a58e1e3420:1] NF removed (../lib/sbi/nnrf-handler.c:750)
04/12 15:16:59.364: [pfcp] WARNING: [5] LOCAL  No Reponse. Give up! for step 1 type 1 peer [127.0.0.4]:8805 (../lib/pfcp/xact.c:599)
04/12 15:16:59.364: [upf] WARNING: No Heartbeat from SMF [127.0.0.4]:8805 (../src/upf/pfcp-sm.c:329)
04/12 15:16:59.364: [upf] INFO: PFCP de-associated [127.0.0.4]:8805 (../src/upf/pfcp-sm.c:199)
```

So, I've fixed it.