Check ImagingNewDirty return value

[radarhere]

ImagingNewDirty may return NULL

Pillow/src/libImaging/Storage.c

Lines 623 to 641 in d12af6d

	Imaging
	ImagingNewDirty(const ModeID mode, int xsize, int ysize) {
	if (ImagingDefaultArena.use_block_allocator) {
	return ImagingNewBlock(mode, xsize, ysize);
	}
	return ImagingNewInternal(mode, xsize, ysize, 1);
	}
	Imaging
	ImagingNewBlock(const ModeID mode, int xsize, int ysize) {
	Imaging im;
	if (xsize < 0 || ysize < 0) {
	return (Imaging)ImagingError_ValueError("bad image size");
	}
	im = ImagingNewPrologue(mode, xsize, ysize);
	if (!im) {
	return NULL;