avformat/iff: More completely check body_size

Fixes: infinite loop
Fixes: 26485/clusterfuzz-testcase-minimized-ffmpeg_dem_IFF_fuzzer-5126561373880320

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

libavformat/iff.c

@@ -839,7 +839,7 @@ static int iff_read_packet(AVFormatContext *s,
         } else if (st->codecpar->codec_tag == ID_DST) {
             return read_dst_frame(s, pkt);
         } else {
-            if (iff->body_size > INT_MAX)
+            if (iff->body_size > INT_MAX || !iff->body_size)
                 return AVERROR_INVALIDDATA;
             ret = av_get_packet(pb, pkt, iff->body_size);
         }
@@ -875,6 +875,8 @@ static int iff_read_packet(AVFormatContext *s,
             pkt->flags |= AV_PKT_FLAG_KEY;
     } else if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO &&
                st->codecpar->codec_tag  != ID_ANIM) {
+        if (iff->body_size > INT_MAX || !iff->body_size)
+            return AVERROR_INVALIDDATA;
         ret = av_get_packet(pb, pkt, iff->body_size);
         pkt->pos = pos;
         if (pos == iff->body_pos)