BUG: Fix for crash in vtkImageResliceMask (#3301)

* Force the output image and mask extent to be the same to avoid the possibility of mismatched extents
* Add a NULL-pointer check after the backgroundMaskPtr setting to prevent application crash (worst case it causes partial screen refresh)

From: Andras Lasso <lasso@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23177 3bd1e089-480b-0410-8dfb-8563597acbee