Fix buffer overflow when saving compressed DDS images

[radarhere]

No description provided.

[marbens-arch]

PoC made by the discoverer (not me):

import argparse
from PIL import Image

parser = argparse.ArgumentParser()
parser.add_argument('image')

args = parser.parse_args()

im = Image.open(args.image)
im = im.convert('RGBA')

img440 = im.resize((440,440))

img440.save('image.dds', 'DDS', pixel_format='DXT1')

python3 pillow_bcn_bug.py image.png (where image.png is the path to the bird picture) outputs:

double free or corruption (!prev)
Aborted (core dumped)