handle zero-length password gracefully

Author: musicsnobj

zxcvbn/scoring.py

@@ -173,6 +173,10 @@ def make_bruteforce_match(i, j):
     # helper: step backwards through optimal.m starting at the end,
     # constructing the final optimal match sequence.
     def unwind(n):
+        if n == 0:
+            # return empty list for zero-length password
+            return []
+
         optimal_match_sequence = []
         k = n - 1
         # find the final best sequence length and score