FS-7513: fix segfault on conference hangup

src/mod/applications/mod_conference/mod_conference.c

@@ -6591,7 +6591,6 @@ static void conference_loop_output(conference_member_t *member)
 		if (member->video_muxing_write_thread) {
 			switch_queue_push(member->mux_out_queue, NULL);
 			switch_thread_join(&st, member->video_muxing_write_thread);
-			switch_frame_buffer_destroy(&member->fb);
 		}
 	}
 
@@ -11754,6 +11753,10 @@ SWITCH_STANDARD_APP(conference_function)
 	switch_buffer_destroy(&member.audio_buffer);
 	switch_buffer_destroy(&member.mux_buffer);
 
+	if (member.fb) {
+		switch_frame_buffer_destroy(&member.fb);
+	}
+
 	if (conference) {
 		switch_mutex_lock(conference->mutex);
 		if (switch_test_flag(conference, CFLAG_DYNAMIC) && conference->count == 0) {