Handle empty string map keys.

src/vm/wren_value.c

@@ -368,11 +368,14 @@ static uint32_t hashObject(Obj* object)
       // towards the prefix or suffix of the string. So sample up to eight
       // characters spread throughout the string.
       // TODO: Tune this.
-      uint32_t step = 1 + 7 / string->length;
-      for (uint32_t i = 0; i < string->length; i += step)
+      if (string->length > 0)
       {
-        hash ^= string->value[i];
-        hash *= 16777619;
+        uint32_t step = 1 + 7 / string->length;
+        for (uint32_t i = 0; i < string->length; i += step)
+        {
+          hash ^= string->value[i];
+          hash *= 16777619;
+        }
       }
 
       return hash;