Update ext/mbstring/mbstring.c

Co-authored-by: Niels Dossche <7771979+nielsdos@users.noreply.github.com>

Author: youkidearitai

ext/mbstring/mbstring.c

@@ -2974,7 +2974,7 @@ static zend_string* trim_each_wchar(zend_string *str, zend_string *what, MB_TRIM
 
 	HashTable what_ht;
 	zval val;
-	ZVAL_EMPTY_STRING(&val);
+	ZVAL_TRUE(&val);
 	zend_hash_init(&what_ht, what_len, NULL, NULL, false);
 
 	while (what_len) {