obexd: Fix possible crash when processing session callback
obexd: Fix possible crash when processing session callback

If the callback removes the pending data it cause this:

==20639== Invalid read of size 4
==20639==    at 0x80553E9: free_pending (session.c:112)
==20639==    by 0x8056C83: session_request_reply (session.c:837)
==20639==    by 0x412F7E0: ??? (in /lib/libdbus-1.so.3.5.2)
==20639==    by 0x411D975: ??? (in /lib/libdbus-1.so.3.5.2)
==20639==    by 0x4120B81: dbus_connection_dispatch (in /lib/libdbus-1.so.3.5.2)
==20639==    by 0x804C27F: message_dispatch (mainloop.c:80)
==20639==    by 0x407EFCB: ??? (in /lib/libglib-2.0.so.0.2600.1)
==20639==    by 0x407E854: g_main_context_dispatch (in /lib/libglib-2.0.so.0.2600.1)
==20639==    by 0x4082667: ??? (in /lib/libglib-2.0.so.0.2600.1)
==20639==    by 0x4082BA6: g_main_loop_run (in /lib/libglib-2.0.so.0.2600.1)
==20639==    by 0x8055171: main (main.c:625)
==20639==  Address 0x4363c88 is 0 bytes inside a block of size 12 free'd
==20639==    at 0x40257ED: free (vg_replace_malloc.c:366)
==20639==    by 0x4087485: g_free (in /lib/libglib-2.0.so.0.2600.1)
==20639==    by 0x80553FE: free_pending (session.c:115)
==20639==    by 0x805543C: agent_free (session.c:127)
==20639==    by 0x80566A6: session_free (session.c:149)
==20639==    by 0x8056BCA: session_terminate_transfer (session.c:914)
==20639==    by 0x8056F61: session_prepare_put (session.c:1397)
==20639==    by 0x8056C74: session_request_reply (session.c:835)
==20639==    by 0x412F7E0: ??? (in /lib/libdbus-1.so.3.5.2)
==20639==    by 0x411D975: ??? (in /lib/libdbus-1.so.3.5.2)
==20639==    by 0x4120B81: dbus_connection_dispatch (in /lib/libdbus-1.so.3.5.2)
==20639==    by 0x804C27F: message_dispatch (mainloop.c:80)

To fix this agent->pending is now reset to NULL before calling the
callback, so even if the session is terminated it won't cause a free to
pending data, which is fine since it is latter freed on callback return.

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