avformat/apngdec: Return error for incomplete header

If avio_read() could read anything, it returns the number of bytes read,
even if it could not read as much as the caller desired.
apng_read_header() only checked the return value of its avio_read() calls
for being negative and this meant that it was possible for an incomplete
header to not be detected. The return value of the last successfull call
has been returned instead. This commit changes this.

Fixes: OOM
Fixes: 26608/clusterfuzz-testcase-minimized-ffmpeg_dem_APNG_fuzzer-4839491644424192

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/apngdec.c

@@ -138,7 +138,7 @@ static int append_extradata(AVCodecParameters *par, AVIOContext *pb, int len)
     par->extradata = new_extradata;
     par->extradata_size = new_size;
 
-    if ((ret = avio_read(pb, par->extradata + previous_size, len)) < 0)
+    if ((ret = ffio_read_size(pb, par->extradata + previous_size, len)) < 0)
         return ret;
 
     return previous_size;
@@ -185,10 +185,10 @@ static int apng_read_header(AVFormatContext *s)
     AV_WL32(st->codecpar->extradata+4,  tag);
     AV_WB32(st->codecpar->extradata+8,  st->codecpar->width);
     AV_WB32(st->codecpar->extradata+12, st->codecpar->height);
-    if ((ret = avio_read(pb, st->codecpar->extradata+16, 9)) < 0)
-        goto fail;
+    if ((ret = ffio_read_size(pb, st->codecpar->extradata + 16, 9)) < 0)
+        return ret;
 
-    while (!avio_feof(pb)) {
+    while (1) {
         if (acTL_found && ctx->num_play != 1) {
             int64_t size   = avio_size(pb);
             int64_t offset = avio_tell(pb);