Clear isOpenError flag on successful open call

Clear the isOpenError flag in IBase driver if the QIBaseDriver::open()
call was successful, otherwise a previous, unsuccessful open() call would
block any further QSqlQuery::exec() calls on this database connection.

Task-number: QTBUG-13435
Change-Id: Idc64e28cd63805a13f208702ec87dc1bf6b98798
[ChangeLog][QtSql][QIBASE] Fixed the internal state of IBase driver after a failed open call
Reviewed-by: Mark Brand <mabrand@mabrand.nl>