string.c: use rb_check_string_type

* string.c (rb_str_cmp_m): use rb_check_string_type for check and
  conversion, instead of calling the conversion method directly.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57965 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -3181,19 +3181,11 @@ static VALUE
 rb_str_cmp_m(VALUE str1, VALUE str2)
 {
     int result;
-
-    if (!RB_TYPE_P(str2, T_STRING)) {
-	VALUE tmp = rb_check_funcall(str2, idTo_str, 0, 0);
-	if (RB_TYPE_P(tmp, T_STRING)) {
-	    result = rb_str_cmp(str1, tmp);
-	}
-	else {
-	    return rb_invcmp(str1, str2);
-	}
-    }
-    else {
-	result = rb_str_cmp(str1, str2);
+    VALUE s = rb_check_string_type(str2);
+    if (NIL_P(s)) {
+	return rb_invcmp(str1, str2);
     }
+    result = rb_str_cmp(str1, s);
     return INT2FIX(result);
 }