Do not try to create invalid sized XPixmaps

If a visible plugin is resized to an empty size we request an empty
sized xpixmap which is illegal.

Instead hide and only free the exiting drawable.

Task-number: QTBUG-38541
Change-Id: I2cb3d221d656b1b18391c479168221bcd0aa73b1
Reviewed-by: Michael Bruning <michael.bruning@digia.com>