gdbus: Fix segfault when D-Bus daemon exits

gdbus: Fix segfault when D-Bus daemon exits

Fix this crash if D-Bus exits while the client is still connected to it:

==5570== Invalid read of size 1
==5570==    at 0x402D28E: strcmp (in
/usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==5570==    by 0x4070E22: g_str_equal (ghash.c:1704)
==5570==    by 0x8055F61: message_filter (client.c:1123)
==5570==    by 0x4141500: dbus_connection_dispatch (in
/lib/i386-linux-gnu/libdbus-1.so.3.5.8)
==5570==    by 0x80506F7: message_dispatch (mainloop.c:76)
==5570==    by 0x4081A7E: g_timeout_dispatch (gmain.c:3882)
==5570==    by 0x4080D85: g_main_context_dispatch (gmain.c:2539)
==5570==    by 0x4081124: g_main_context_iterate.isra.21 (gmain.c:3146)
==5570==    by 0x408156A: g_main_loop_run (gmain.c:3340)
==5570==    by 0x41BF4D2: (below main) (libc-start.c:226)
==5570==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==5570==
==5570==