fix: src\string.c(497): warning C4244: '=': conversion from 'mrb_int' to 'long', possible loss of data

src/string.c

@@ -488,7 +488,7 @@ check_frozen(mrb_state *mrb, struct RString *s)
 static mrb_value
 str_replace(mrb_state *mrb, struct RString *s1, struct RString *s2)
 {
-  long len;
+  mrb_int len;
 
   check_frozen(mrb, s1);
   if (s1 == s2) return mrb_obj_value(s1);