audio/AVDTP: Fix crash after disconnecting
audio/AVDTP: Fix crash after disconnecting

If the session is disconnected while process_discover is pending the
source id is not removed causing the following crash:
      at 0x414C08: finalize_discovery (avdtp.c:1050)
      by 0x414C5A: process_discover (avdtp.c:3346)
      by 0x3D46047E05: g_main_context_dispatch (in /usr/lib64/libglib-2.0.so.0.3600.3)
      by 0x3D46048157: ??? (in /usr/lib64/libglib-2.0.so.0.3600.3)
      by 0x3D46048559: g_main_loop_run (in /usr/lib64/libglib-2.0.so.0.3600.3)
      by 0x40A3B6: main (main.c:595)
    Address 0x5e25de8 is 1,144 bytes inside a block of size 1,176 free'd
    at 0x4A074C4: free (in
    /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
      by 0x3D4604D9AE: g_free (in /usr/lib64/libglib-2.0.so.0.3600.3)
      by 0x4152F2: connection_lost (avdtp.c:1206)
      by 0x4162C4: cancel_request (avdtp.c:2662)
      by 0x4164BD: request_timeout (avdtp.c:2672)
      by 0x3D46048962: ??? (in /usr/lib64/libglib-2.0.so.0.3600.3)
      by 0x3D46047E05: g_main_context_dispatch (in /usr/lib64/libglib-2.0.so.0.3600.3)
      by 0x3D46048157: ??? (in /usr/lib64/libglib-2.0.so.0.3600.3)
      by 0x3D46048559: g_main_loop_run (in /usr/lib64/libglib-2.0.so.0.3600.3)
      by 0x40A3B6: main (main.c:595)

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