deps/libff: Fix incorrect timer triggering (twice)

Sometimes the timer will be triggered twice in one request.  GIF files
can be rendered in wrong speed.

Closes jp9000/obs-studio#513

deps/libff/libff/ff-timer.c

@@ -51,6 +51,9 @@ static void *timer_thread(void *opaque)
 						- current_time));
 			}
 
+			pthread_mutex_unlock(&timer->mutex);
+			continue;
+
 			// we can be woken up merely to set a sooner wake time
 
 		} else {