Pillow: Out-of-bounds access in _imagingmorph on a zero-width image

### Summary A PPM or PGM file can declare a width of 0, which Pillow accepts, producing an image of size `(0, N)`. Passing that image to `ImageMorph.MorphOp.apply()` makes the native morphology code index a row buffer at `outrow[width - 1]`, which is `outrow[-1]` when the width is 0. The process crashes. ### Details The PPM parser reads width and height with no lower-bound check: - `src/PIL/PpmImagePlugin.py:119` The native `apply()` writes the row edges with no guard that the width is positive: - `src/_imagingmorph.c:81` sets `outrow[0] = outrow[width - 1] = 0;` `match()` and `get_on_pixels()` use the same edge-indexing pattern. ### PoC ```python from PIL import Image, ImageMorph # A width-0 image (also producible from a crafted PPM header). apply() indexes # outrow[width - 1] == outrow[-1] in native code. im = Image.new("L", (0, 5)) ImageMorph.MorphOp(op_name="dilation4").apply(im) ``` Run it: ``` $ python 03_poc.py Segmentation fault (core dumped) ``` Observed on a source build of commit `3e2ab96`. Under AddressSanitizer: ``` ==ERROR: AddressSanitizer: SEGV on unknown address (pc ... in apply) #0 apply src/_imagingmorph.c:81 SUMMARY: AddressSanitizer: SEGV src/_imagingmorph.c:81 in apply ``` With width 0 the `outrow[width - 1]` write lands on an unmapped page, so it surfaces as a SEGV pinned to the edge-write line rather than a redzone overflow. ### Impact Out-of-bounds write in a C extension. Affects any application that runs a morphology operation on an image opened from untrusted input. Result is a crash and possible memory corruption.