Add check for regex match

Author: toolboc

YoutubeExtractor/YoutubeExtractor/Decipherer.cs

@@ -122,7 +122,7 @@ private static string FindMatch(string text, string regexp)
             Regex rgx = new Regex(regexp);
             var matches = rgx.Matches(text);
 
-            return matches[0].Groups[1].Value;
+            return matches.Count > 0 ? matches[0].Groups[1].Value : string.Empty;
         }
     }
 }