Fix access of tmpStorage in QODBCResult::exec().

Instead of using a list and appending / popping of elements, use
a vector of constant size and access via index to avoid
bookkeeping errors.

Task-number: QTBUG-37831

Change-Id: Icb5a182626c63e693b04daaf7a2f70997d9aeae1
Reviewed-by: Mark Brand <mabrand@mabrand.nl>
(cherry picked from qtbase/d5ff16bc8ebf0f17fae3a28f08018db26567e81c)
Reviewed-by: Andy Shaw <andy.shaw@digia.com>