Fix crashes when BLE rendezvous disconnects unexpectedly

[mspang]

In BLE::OnBleEndPointConnectionClosed, the endpoint is already closed.
We try to close it again in the Transport::BLE destructor, which can crash.
Clear the endpoint in OnBleEndPointConnectionClosed to avoid this case.

In addition, the OnRendezvousError callback in the typical case stops
the rendezvous process which immediately destroys the transport. We
therefore can't call OnRendezvousConnectionClosed afterwards; |this| is
already freed and we just need to return.

[github-actions]

Size increase report for "nrfconnect-example-build" from b430c42

File	Section	File	VM
chip-lock.elf	shell_root_cmds_sections	-8	-8
chip-lock.elf	text	-8	-8
chip-lighting.elf	shell_root_cmds_sections	-8	-8
chip-lighting.elf	text	-8	-8

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-lock.elf and ./pull_artifact/chip-lock.elf:

sections,vmsize,filesize
.debug_loc,0,28
.debug_info,0,-1
.debug_line,0,-1
shell_root_cmds_sections,-8,-8
text,-8,-8
.debug_abbrev,0,-10
.debug_frame,0,-16

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-lighting.elf and ./pull_artifact/chip-lighting.elf:

sections,vmsize,filesize
.debug_loc,0,28
.debug_info,0,-1
.debug_line,0,-1
shell_root_cmds_sections,-8,-8
text,-8,-8
.debug_abbrev,0,-10
.debug_frame,0,-16