gdbus: Fix wrong signal handler match

gdbus: Fix wrong signal handler match

When we add a signal handler with g_dbus_add_signal_watch(), this
function tries to multiplex the matches added in libdbus by checking
if there's a previous filter_data with the same fields. However, if the
field is NULL it accepts as being the same. The result is that the
following watches will use the same filter data:

watch1 = g_dbus_add_signal_watch(conn, BUS_NAME, NULL, iface, member,
						cb1, data1, NULL);
watch2 = g_dbus_add_signal_watch(conn, BUS_NAME, "/path2", iface, member,
						cb2, data2, NULL);
watch3 = g_dbus_add_signal_watch(conn, BUS_NAME, "/path3", iface, member,
						cb3, data3, NULL);

The result is that when a signal arrives with path == "/path2", all 3
callbacks above will be called, with the same signal delivered to all of
them.

Another problem is that, if we invert the calls like below, only signals
to cb1 will never be trigerred, nonetheless it used path == NULL.

watch2 = g_dbus_add_signal_watch(conn, BUS_NAME, "/path2", iface, member,
						cb2, data2, NULL);
watch1 = g_dbus_add_signal_watch(conn, BUS_NAME, NULL, iface, member,
						cb1, data1, NULL);
watch3 = g_dbus_add_signal_watch(conn, BUS_NAME, "/path3", iface, member,
						cb3, data3, NULL);

This is fixed by not multiplexing the matchs with filter data if any of
the fields are different, including being NULL. When a signal arrives,
if a field is NULL we accept it as a match, but not when adding the
signal handler.