lavfi/alphaextract: fix assignment of invalid size value to memcpy in case linesize < 0

Fix crash, for example in the command:
ffmpeg -f image2 -vcodec pgmyuv -i tests/vsynth1/%02d.pgm \
  -vf "[in]format=yuv420p,split,alphamerge,split[o3][o4];
       [o4]vflip,alphaextract[alpha];[o3][alpha]alphamerge[out]" \
  -vcodec rawvideo -f nut md5: