Fix GH-17400: bindtextdomain segfault with UTF-16 domain value.

The provided domain could be a non ascii value even if not supposed to,
in the error reported case was of 4 code points long but domain is "empty" leading to
a NULL return. It worked up to 8.3 "by accident" before the zend_string
conversion and check prior for emptiness.

Author: devnexen

ext/gettext/gettext.c

@@ -212,6 +212,9 @@ PHP_FUNCTION(bindtextdomain)
 	}
 
 	retval = bindtextdomain(ZSTR_VAL(domain), dir_name);
+	if (retval == NULL) {
+		RETURN_FALSE;
+	}
 
 	RETURN_STRING(retval);
 }