avcodec/pngdec: Check that previous_picture has same w/h/format

Fixes: out of array access
Fixes: 15540/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_APNG_fuzzer-5684905029140480

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pngdec.c

@@ -1391,6 +1391,9 @@ exit_loop:
             if (CONFIG_PNG_DECODER && avctx->codec_id != AV_CODEC_ID_APNG)
                 handle_p_frame_png(s, p);
             else if (CONFIG_APNG_DECODER &&
+                     s->previous_picture.f->width == p->width  &&
+                     s->previous_picture.f->height== p->height &&
+                     s->previous_picture.f->format== p->format &&
                      avctx->codec_id == AV_CODEC_ID_APNG &&
                      (ret = handle_p_frame_apng(avctx, s, p)) < 0)
                 goto fail;