avcodec/pngdec: Use ff_set_dimensions()

Fixes OOM
Fixes: 1314/clusterfuzz-testcase-minimized-4621997222920192

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
