Return None when a matrix cannot be applied.

naive-nlu/tree_nlu/parsing.py

@@ -130,6 +130,8 @@ def apply_remix(tokens, remix):
     rebuilt = []
     for i in remix:
         if isinstance(i, int):
+            if i >= len(tokens):
+                return None
             rebuilt.append(tokens[i])
         else:
             assert(isinstance(i, str))