SQL: Change in QSqlResultPrivate::positionalToNamedBinding()

SQL: Change in QSqlResultPrivate::positionalToNamedBinding()

With the change cff46983a, prepared queries can now store a
named paramater to be used more than once.

When using ?, thus positional binding, there is no named
parameter, thus there is no need to store it.

When prepare is called from a query with ?, it currently
causes an error when the feature QSqlDriver::NamedPlaceholders
is true. Because holders values are called while holders is
actually empty.

QSqlDriver::NamedPlaceholders is true for QOCI plugin only
but the problem is independant of the plugin used.

Adding a test case with a test driver to make the test runnable
without Oracle installed.

Change-Id: I6d7491f7e09a7b62d2d4d216b40fedd67e927e27
Reviewed-by: Matt Newell <newellm@blur.com>
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Mark Brand <mabrand@mabrand.nl>