Return true for QSqlQuery::isNull on non-existent fields.

This fixes the tst_QSqlQuery::isNull test case when run against a
MySQL database driver.

Change-Id: I8248ba956472bae97a64247594055e6f02840557
Reviewed-by: Mark Brand <mabrand@mabrand.nl>

src/sql/drivers/mysql/qsql_mysql.cpp

@@ -670,6 +670,8 @@ QVariant QMYSQLResult::data(int field)
 
 bool QMYSQLResult::isNull(int field)
 {
+   if (field < 0 || field >= d->fields.count())
+       return true;
    if (d->preparedQuery)
        return d->fields.at(field).nullIndicator;
    else