ext/standard: handle html entities empty string before processing (php#19220)

Author: xepozz

ext/standard/html.c

@@ -1356,6 +1356,9 @@ static void php_html_entities(INTERNAL_FUNCTION_PARAMETERS, int all)
 		Z_PARAM_BOOL(double_encode);
 	ZEND_PARSE_PARAMETERS_END();
 
+	if (ZSTR_LEN(str) == 0) {
+		RETURN_EMPTY_STRING();
+	}
 	replaced = php_escape_html_entities_ex(
 		(unsigned char*)ZSTR_VAL(str), ZSTR_LEN(str), all, (int) flags,
 		hint_charset ? ZSTR_VAL(hint_charset) : NULL, double_encode, /* quiet */ 0);