gdbus/watch: Fix crash when disconnecting from D-Bus

gdbus/watch: Fix crash when disconnecting from D-Bus

When disconnecting from D-Bus a message could be recieved with no sender:
Invalid read of size 1
   at 0x4A09EE1: strcmp (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x3B03C386B8: g_str_equal (in /usr/lib64/libglib-2.0.so.0.3600.3)
   by 0x4065D6: message_filter (watch.c:529)
   by 0x3B0700F9E5: dbus_connection_dispatch (in /usr/lib64/libdbus-1.so.3.7.4)
   by 0x4052E7: message_dispatch (mainloop.c:76)
   by 0x3B03C48962: ??? (in /usr/lib64/libglib-2.0.so.0.3600.3)
   by 0x3B03C47E05: g_main_context_dispatch (in /usr/lib64/libglib-2.0.so.0.3600.3)
   by 0x3B03C48157: ??? (in /usr/lib64/libglib-2.0.so.0.3600.3)
   by 0x3B03C48559: g_main_loop_run (in /usr/lib64/libglib-2.0.so.0.3600.3)
   by 0x4038C5: client_proxy_removed (test-gdbus-client.c:902)
   by 0x3B03C6B566: ??? (in /usr/lib64/libglib-2.0.so.0.3600.3)
   by 0x3B03C6B6E5: ??? (in /usr/lib64/libglib-2.0.so.0.3600.3)
 Address 0x0 is not stack'd, malloc'd or (recently) free'd