avcodec/pngdec: dont skip/read chunk twice

Fixes: out of array access
Fixes: 48567/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PNG_fuzzer-6668158952144896.fuzz

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit df1a38d520)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pngdec.c

@@ -1231,6 +1231,7 @@ static int decode_frame_common(AVCodecContext *avctx, PNGDecContext *s,
                 }
                 av_log(avctx, AV_LOG_ERROR, ", skipping\n");
                 bytestream2_skip(&s->gb, length + 8); /* tag */
+                continue;
             }
         }
         tag = bytestream2_get_le32(&s->gb);