A malformed bmp file header could specify a negative color table
size. The bmp handler would then return a QImage that claimed to be
valid, but actually was invalid, having an empty color table. This
would cause crash later, e.g. when attempting to paint it.

Change-Id: I7df7c40867557a82dbcee44c7de061226ff232c0
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Richard J. Moore <rich@kde.org>