Set pointer focus to null when surface is destroyed.

We must do something otherwise the next inputDevice->mouseFocus() call
will dereference an already deleted instance.

Change-Id: I3ac92a37715f63c6dc43191968a8da51befbccde
Sanity-Review: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>