Fix Firebird driver

Author: Girgias

ext/pdo_firebird/firebird_driver.c

@@ -671,7 +671,7 @@ static zend_string* firebird_handle_quoter(pdo_dbh_t *dbh, const zend_string *un
 	*c++ = '\'';
 
 	/* foreach (chunk that ends in a quote) */
-	for (l = unquoted; (r = strchr(l,'\'')); l = r+1) {
+	for (l = ZSTR_VAL(unquoted); (r = strchr(l,'\'')); l = r+1) {
 		strncpy(c, l, r-l+1);
 		c += (r-l+1);
 		/* add the second quote */