ext/mbstring: fix new_value length check

Commit 8bbd095 added a check rejecting empty strings; in the
merge commiot 379d9a1 however it was changed to a NULL check,
one that did not make sense because ZSTR_VAL() is guaranteed to never
be NULL; the length check was accidently removed by that merge commit.

This bug was found by GCC's -Waddress warning:

 ext/mbstring/mbstring.c:748:27: warning: the comparison will always evaluate as ‘true’ for the address of ‘val’ will never be NULL [-Waddress]
   748 |         if (!new_value || !ZSTR_VAL(new_value)) {
       |                           ^

Author: MaxKellermann

ext/mbstring/mbstring.c

@@ -745,7 +745,7 @@ static PHP_INI_MH(OnUpdate_mbstring_http_input)
 		php_error_docref("ref.mbstring", E_DEPRECATED, "Use of mbstring.http_input is deprecated");
 	}
 
-	if (!new_value || !ZSTR_VAL(new_value)) {
+	if (!new_value || !ZSTR_LEN(new_value)) {
 		const char *encoding = php_get_input_encoding();
 		MBSTRG(http_input_set) = 0;
 		_php_mb_ini_mbstring_http_input_set(encoding, strlen(encoding));