Crash on stress test

[krizhanovsky]

Crash occurs after the recent fix for #1217 . It's 100% reproducable, just use default test config and run:

./run_tests.py -n tls.test_tls_stress

[   18.513152] BUG: unable to handle kernel NULL pointer dereference at 00000000000002a6
[   18.514599] IP: tfw_tls_encrypt+0x47c/0x680 [tempesta_fw]
[   18.515579] PGD 800000002f051067 P4D 800000002f051067 PUD 0
[   18.516583] Oops: 0000 [#1] SMP PTI
[   18.517210] Modules linked in: tempesta_fw(O) tempesta_db(O) tempesta_tls(O) tempesta_lib(O) binfmt_misc bochs_drm ttm drm_kms_helper drm crct10dif_pclmul ppdev fb_sys_fops syscopyarea sysfillrect pcspkr serio_raw sg parport_pc parport sysimgblt button ip_tables x_tables ext4 crc16 mbcache jbd2 fscrypto sr_mod cdrom sd_mod ata_generic ata_piix psmouse libata e1000 scsi_mod i2c_piix4
[   18.523208] CPU: 1 PID: 1036 Comm: wrk Tainted: G           O    4.14.32-kdump+ #142
[   18.524671] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-2.fc30 04/01/2014
[   18.526238] task: ffff954e2f07b1c0 task.stack: ffff954e48210000
[   18.527296] RIP: 0010:tfw_tls_encrypt+0x47c/0x680 [tempesta_fw]
[   18.528340] RSP: 0000:ffff954e7fd03b48 EFLAGS: 00010283
[   18.529271] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000004
[   18.530517] RDX: 0000000000000000 RSI: ffff954e400dabd3 RDI: 0000000000000286
[   18.531830] RBP: ffff954e400daa00 R08: 0000000000000010 R09: fffffffffffffffd
[   18.533137] R10: ffff954e7fd03aa0 R11: ffffe5cc80bcec00 R12: ffff954e7fd03ba8
[   18.534389] R13: 0000000000000007 R14: ffff954e5cb9ca78 R15: ffff954e5cb9c748
[   18.536351] FS:  00007fdb1b371700(0000) GS:ffff954e7fd00000(0000) knlGS:0000000000000000
[   18.537996] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   18.539129] CR2: 00000000000002a6 CR3: 0000000048208006 CR4: 00000000003606e0
[   18.540578] Call Trace:
[   18.541050]  <IRQ>
[   18.541483]  ? task_tick_fair+0x515/0x610
[   18.542292]  tcp_write_xmit+0x41f/0x11c0
[   18.543036]  __tcp_push_pending_frames+0x28/0x90
[   18.543908]  ss_tx_action+0x4e7/0x570 [tempesta_fw]
[   18.544789]  ? check_preempt_curr+0x7e/0x90
[   18.545553]  ? ttwu_do_wakeup.isra.86+0xf/0xe0
[   18.546376]  net_tx_action+0x8d/0x1b0
[   18.547070]  __do_softirq+0x104/0x297
[   18.548837]  irq_exit+0xa3/0xb0
[   18.550316]  call_function_single_interrupt+0x7a/0x80
[   18.552294]  </IRQ>
[   18.553560] RIP: 0033:0x556e873a6bf2
[   18.555308] RSP: 002b:00007fdb1b370cc0 EFLAGS: 00000202 ORIG_RAX: ffffffffffffff04
[   18.557655] RAX: 0000000040c98400 RBX: 0000000040c8f378 RCX: 0000000065736e6f
[   18.559813] RDX: 00007fdb1b370cd8 RSI: 0000000040c8f448 RDI: 0000000040c8f378
[   18.561977] RBP: 0000556e875fd570 R08: 0000000040c8f3b8 R09: 0000000040c8f378
[   18.564125] R10: 000000000000148a R11: 00007ffe7e5bb080 R12: 0000000040c8f448
[   18.567398] R13: 00000000fffffffd R14: 00007fdb14004aa8 R15: 00007fdb14004b69
[   18.569775] Code: 89 f7 41 88 87 af 03 00 00 e8 d1 21 f9 ff 85 c0 0f 84 e5 00 00 00 41 c6 87 30 03 00 00 00 4c 3b 64 24 38 73 2c 89 c3 49 8b 3c 24 <48> 8b 47 20 48 8d 50 ff a8 01 48 0f 45 fa f0 ff 4f 1c 75 05 e8