Add some sanity and bounds checking

Since QImage does sanity checking for overflows and stuff wrt.
dimensions and depth, check for QImage::isNull() as early as possible to
see if there's some funky business going on.

Also tried to add some checks wherever we wrote to "raw" memory.

Unit tests pass, and tested converting some files from
https://samples.ffmpeg.org/image-samples/ to pngs, and that seemed to
work.

Reviewed By: aacid

Differential Revision: https://phabricator.kde.org/D24367