fix IndexError when empty cassette throws CannotOverwriteCassetteException

vcr/cassette.py

@@ -311,7 +311,7 @@ class Cassette(object):
         best_matches.sort(key=lambda t: t[0], reverse=True)
         # Get the first best matches (multiple if equal matches)
         final_best_matches = []
-        previous_nb_success = best_matches[0][0]
+        previous_nb_success = 0 
         for best_match in best_matches:
             nb_success = best_match[0]
             # Do not keep matches that have 0 successes,