audio/avdtp: Fix crash at NULL cb of discover_callback
audio/avdtp: Fix crash at NULL cb of discover_callback

This fixes a crash with below trace that a discover_callback
object has a NULL cb function pointer being triggered, which
may due to an error happened in avdtp_discover() and left it
unset.

0x00000000 in ?? ()
0xb6f5606e in finalize_discovery (err=22, session=0xb7514a00)
  at profiles/audio/avdtp.c:1043
connection_lost (session=session@entry=0xb7514a00, err=err@entry=22)
  at profiles/audio/avdtp.c:1143
0xb6f57cf6 in avdtp_connect_cb (chan=<optimized out>, err=0xb7514410,
  user_data=0xb7514a00) at profiles/audio/avdtp.c:2373
0xb6f723e4 in accept_cb (io=0xb7516a10, cond=<optimized out>,
  user_data=0xb7516b48) at btio/btio.c:203
0xb6e6cf10 in ?? ()

mapfau/bluez,pkarasev3/bluez,silent-snowman/bluez,silent-snowman/bluez,silent-snowman/bluez,mapfau/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,mapfau/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,ComputeCycles/bluez,ComputeCycles/bluez,silent-snowman/bluez,pkarasev3/bluez,pkarasev3/bluez,mapfau/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez