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>

src/sql/drivers/ibase/qsql_ibase.cpp

@@ -1530,6 +1530,7 @@ bool QIBaseDriver::open(const QString & db,
     }
 
     setOpen(true);
+    setOpenError(false);
     return true;
 }