obexd/session: Fix crash when transport is disconnected

obexd/session: Fix crash when transport is disconnected

When transport is disconnected unexpectedly it can cause the following
crash:
gobex-DEBUG: gobex/gobex.c:g_obex_send_internal() The transport is not connected
Invalid read of size 8
   at 0x42662E: session_process_queue (session.c:789)
   by 0x42668F: session_process (session.c:719)
   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 0x40D5FC: main (main.c:319)
 Address 0x5086760 is 32 bytes inside a block of size 56 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 0x426146: session_process_setpath (session.c:1063)
   by 0x426629: session_process_queue (session.c:786)
   by 0x42668F: session_process (session.c:719)
   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 0x40D5FC: main (main.c:319)