obs-transitions: Only check matte duration if matte exists

The code within this block was being called even when there is no matte
source present. Caught by null pointer checks in the log.

plugins/obs-transitions/transition-stinger.c

@@ -439,7 +439,7 @@ static void stinger_transition_start(void *data)
 		s->transition_a_mul = (1.0f / s->transition_point);
 		s->transition_b_mul = (1.0f / (1.0f - s->transition_point));
 
-		if (s->track_matte_enabled) {
+		if (s->track_matte_enabled && s->matte_source) {
 			proc_handler_call(matte_ph, "get_duration", &cd);
 			uint64_t tm_duration_ns =
 				(uint64_t)calldata_int(&cd, "duration");