avcodec/pngdec: Do not pass AVFrame into global header decode

The global header should not contain a frame, and decoding it
would result in leaks

Fixes: memleak
Fixes: 45982/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_APNG_fuzzer-6603443149340672

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

libavcodec/pngdec.c

@@ -734,6 +734,8 @@ static int decode_idat_chunk(AVCodecContext *avctx, PNGDecContext *s,
     int ret;
     size_t byte_depth = s->bit_depth > 8 ? 2 : 1;
 
+    if (!p)
+        return AVERROR_INVALIDDATA;
     if (!(s->hdr_state & PNG_IHDR)) {
         av_log(avctx, AV_LOG_ERROR, "IDAT without IHDR\n");
         return AVERROR_INVALIDDATA;
@@ -1515,6 +1517,9 @@ static int decode_frame_common(AVCodecContext *avctx, PNGDecContext *s,
     }
 exit_loop:
 
+    if (!p)
+        return AVERROR_INVALIDDATA;
+
     if (avctx->codec_id == AV_CODEC_ID_PNG &&
         avctx->skip_frame == AVDISCARD_ALL) {
         return 0;
@@ -1727,7 +1732,7 @@ static int decode_frame_apng(AVCodecContext *avctx, AVFrame *p,
         if ((ret = inflateReset(&s->zstream.zstream)) != Z_OK)
             return AVERROR_EXTERNAL;
         bytestream2_init(&s->gb, avctx->extradata, avctx->extradata_size);
-        if ((ret = decode_frame_common(avctx, s, p, avpkt)) < 0)
+        if ((ret = decode_frame_common(avctx, s, NULL, avpkt)) < 0)
             return ret;
     }