FS-7500: return cng frames when waiting for a key

src/switch_core_media.c

@@ -10430,6 +10430,9 @@ SWITCH_DECLARE(switch_status_t) switch_core_session_read_video_frame(switch_core
 		if (switch_test_flag((*frame), SFF_WAIT_KEY_FRAME)) {
 			switch_core_session_request_video_refresh(session);
 			switch_clear_flag((*frame), SFF_WAIT_KEY_FRAME);
+			*frame = &runtime.dummy_cng_frame;
+			switch_yield(20000);
+			return SWITCH_STATUS_SUCCESS;
 		}
 
 		if (decode_status == SWITCH_STATUS_MORE_DATA || !(*frame)->img) {