gbproxy: Discard UL PTP messages with an unknown BVCI

Currently all PTP messages are in general forwarded to the SGSN even
when the BVCI is not known to the gbproxy. Only if message patching
is active and the peer cannot be determined, a log message is
generated, a STATUS message returned, and the message discarded.
The intention for this was to keep the old gbproxy's behaviour if
patching is disabled. But the code gets much more complex this way.
Another drawback is that when the SGSN returns a corresponding STATUS
message, it cannot be routed to the BSS where the original message
came from.

This patch therefore changes the behaviour to reject BSSGP PTP uplink
messages immediately if the BVCI is not known.

Fixes: Coverity CID 1244240
Ticket: OW#1317
Sponsored-by: On-Waves ehf
gbproxy/test: Don't assign a variable within OSMO_ASSERT (Coverity)

Coverity complains about having side effects in OSMO_ASSERT argument
expressions. This would be an issue in this case, because that
variable is only reference in other OSMO_ASSERT expressions.

Nevertheless this patch changes this to assign the variable outside
of OSMO_ASSERT.

Fixes: Coverity CID 1244239
Sponsored-by: On-Waves ehf
gbproxy: Fix segfault for VTY delete-gbproxy-link

Currently the code segfaults when the link shall be deleted by IMSI
when the IMSI has not been set yet.

This patch adds a NULL check to skip the entry before calling
gsm48_mi_to_string,

Adresses:
Program received signal SIGSEGV, Segmentation fault.
0xb693af77 in gsm48_mi_to_string (string=0xbfffe020 "", str_len=200,
mi=0x0, mi_len=0) at gsm48.c:360
    360         mi_type = mi[0] & GSM_MI_TYPE_MASK;
    str_len=200, mi=0x0, mi_len=0) at gsm48.c:360
    self=0x807c9a0 <delete_gb_link_by_id_cmd>, vty=0xb4303c70,
     argc=3, argv=0xbfffe1c0) at gb_proxy_vty.c:670
...

Sponsored-by: On-Waves ehf