avcodec/pngdec: Clean up on av_frame_ref() failure

Fixes: memleak
Fixes: 3203/clusterfuzz-testcase-minimized-4514553595428864

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