String#replace should check replacing string; fix #3374

This issue was reported by https://hackerone.com/tunz

src/string.c

@@ -519,6 +519,7 @@ str_replace(mrb_state *mrb, struct RString *s1, struct RString *s2)
   long len;
 
   check_frozen(mrb, s1);
+  if (s1 == s2) return mrb_obj_value(s1);
   len = RSTR_LEN(s2);
   if (RSTR_SHARED_P(s1)) {
     str_decref(mrb, s1->as.heap.aux.shared);