FS-7958: [mod_conference] fix race condition causing crash in conference video MCU

src/mod/applications/mod_conference/conference_video.c

@@ -1922,7 +1922,7 @@ void *SWITCH_THREAD_FUNC conference_video_muxing_thread_run(switch_thread_t *thr
 
 			switch_mutex_lock(conference->member_mutex);
 			for (imember = conference->members; imember; imember = imember->next) {
-				if (switch_channel_ready(imember->channel) && conference_utils_member_test_flag(imember, MFLAG_RUNNING) 
+				if (imember->channel && switch_channel_ready(imember->channel) && conference_utils_member_test_flag(imember, MFLAG_RUNNING) 
 					&& imember->canvas_id == canvas->canvas_id) {
 					canvas_count++;
 				}