FS-9314: [mod_conference] fix crash when starting conference in mux mode while…

FS-9314: [mod_conference]  fix crash when starting conference in mux mode while specifying or defaulting to a layout group that does not exist.  We will now fall back to transcode mode in this case.

src/mod/applications/mod_conference/conference_video.c

@@ -1354,6 +1354,10 @@ video_layout_t *conference_video_find_best_layout(conference_obj_t *conference,
 		}
 	}
 
+	if (!lg) {
+		return NULL;
+	}
+
 	for (vlnode = lg->layouts; vlnode; vlnode = vlnode->next) {
 		if (vlnode->vlayout->layers >= (int)count) {
 			break;