Fix #11243: Data corruption with feature simplification

This PR fixes the bug #11243 ( http://hub.qgis.org/issues/11243 ).

It disables the simplification when the reverse transform to calculate the center of current visible extent fails.

The QgsCoordinateTransform::transformBoundingBox() method swap the sign of the maximum longitude value when it is greater than 180.0 as in attached map-data of the issue. Maybe this method needs a fix in order to preserve the sign of the minimun and maximum longitude values of the visible extent.