obexd/client: Fix crash while disconnecting

obexd/client: Fix crash while disconnecting

In case the transport is disconnected while disconnect command is pending
the session is freed on disconnect_complete but disconnect callback is
still valid causing the following crash:

Invalid read of size 4
at 0x42682A: obc_session_ref (session.c:132)
by 0x42797B: obc_session_shutdown (session.c:580)
by 0x4139DA: incoming_data (gobex.c:1406)
by 0x59712A5: g_main_context_dispatch (in /usr/lib64/libglib-2.0.so.0.3800.2)
by 0x5971627: ??? (in /usr/lib64/libglib-2.0.so.0.3800.2)
by 0x5971A39: g_main_loop_run (in /usr/lib64/libglib-2.0.so.0.3800.2)
by 0x40D78C: main (main.c:320)
Address 0x728d814 is 4 bytes inside a block of size 120 free'd
at 0x4C28577: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
by 0x5976F7E: g_free (in /usr/lib64/libglib-2.0.so.0.3800.2)
by 0x4134B9: handle_response (gobex.c:1129)
by 0x4139BD: incoming_data (gobex.c:1403)
by 0x59712A5: g_main_context_dispatch (in /usr/lib64/libglib-2.0.so.0.3800.2)
by 0x5971627: ??? (in /usr/lib64/libglib-2.0.so.0.3800.2)
by 0x5971A39: g_main_loop_run (in /usr/lib64/libglib-2.0.so.0.3800.2)
by 0x40D78C: main (main.c:320)

The patch comes from upstream:
commit ID 765fc36308c8d806a597bb9eae13bdff82e081d8

Change-Id: Id2abc28533d6312c791ec8b9880a4073db529e31
Signed-off-by: Wu Zheng <wu.zheng@intel.com>