Try to fix reading freed memory in eval.c.

This occurred when the null term was reached - the token was freed, but
no error was thrown (it's just the end of the string), so the program
attempted to increment the index using freed token data.

src/eval.c

@@ -209,7 +209,7 @@ liblex_result eval_all(char* string, int index, eval_config* config, ll* matches
           result = ll_append(matches, new_match) == LIBDS_SUCCESS ? LIBLEX_SUCCESS : LIBLEX_MALLOC;
         }
       }
-      if(result == LIBLEX_SUCCESS){
+      if(result == LIBLEX_SUCCESS && string[index] != '\0'){
         index += new_match->to - new_match->from;
       }
     } else {