The HNM 4 video decoder's init function claimed that an allocation
failed if the image dimensions are wrong. This is fixed in this commit:
The dimensions are checked before the allocations are attempted.
The check whether width * height is zero is redundant as
av_image_check_size() already checks for this; it has been removed.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>