Avoid undefined behavior of signed integer overflow; fix #3728

src/string.c

@@ -1521,13 +1521,13 @@ mrb_str_hash(mrb_state *mrb, mrb_value str)
   struct RString *s = mrb_str_ptr(str);
   mrb_int len = RSTR_LEN(s);
   char *p = RSTR_PTR(s);
-  mrb_int key = 0;
+  uint64_t key = 0;
 
   while (len--) {
     key = key*65599 + *p;
     p++;
   }
-  return key + (key>>5);
+  return (mrb_int)(key + (key>>5));
 }
 
 /* 15.2.10.5.20 */