avformat/siff: Basic pkt_size check

Fixes: half of CID1258461 Overflowed constant

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 124a97dd8b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/siff.c

@@ -199,7 +199,10 @@ static int siff_read_packet(AVFormatContext *s, AVPacket *pkt)
         if (c->cur_frame >= c->frames)
             return AVERROR_EOF;
         if (c->curstrm == -1) {
-            c->pktsize = avio_rl32(s->pb) - 4;
+            unsigned pktsize = avio_rl32(s->pb);
+            if (pktsize < 4)
+                return AVERROR_INVALIDDATA;
+            c->pktsize = pktsize - 4;
             c->flags   = avio_rl16(s->pb);
             if (c->flags & VB_HAS_AUDIO && !c->has_audio)
                 return AVERROR_INVALIDDATA;