avformat/oggparseogm: Check lb against psize

No testcase, this was found during code review

Found-by: Matt Wolenetz <wolenetz@google.com>
Reviewed-by: Matt Wolenetz <wolenetz@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 3e7c847aaf)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/oggparseogm.c

@@ -177,6 +177,9 @@ ogm_packet(AVFormatContext *s, int idx)
         os->pflags |= AV_PKT_FLAG_KEY;
 
     lb = ((*p & 2) << 1) | ((*p >> 6) & 3);
+    if (os->psize < lb + 1)
+        return AVERROR_INVALIDDATA;
+
     os->pstart += lb + 1;
     os->psize -= lb + 1;