thumbnailer: fix assertion failure

Sources/VLCMediaThumbnailer.m

@@ -131,6 +131,8 @@ void unlock(void *opaque, void *picture, void *const *p_pixels)
 
     if (!videoTrack) {
         VKLog(@"WARNING: Can't find video track info, skipping file");
+        [_parsingTimeoutTimer invalidate];
+        _parsingTimeoutTimer = nil;
         [self mediaThumbnailingTimedOut];
         return;
     } else {