better handling of GIFs with out of bounds frames
better handling of GIFs with out of bounds frames

The recent reworking of gifload did not check for out of bounds frames
before setting DISPOSE_BACKGROUND.

jcupitt/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips