avformat/wtvdec: Skip too big tags

get_tag() is not designed with negative length in mind;
in this case, it will allocate a very small buffer
(LEN_PRETTY_GUID + 1) and might call avio_get_str16le()
with a negative maxlen (which relies on these parameters
to be signed).

Reviewed-by: Peter Ross <pross@xvid.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/wtvdec.c

@@ -539,7 +539,7 @@ static void parse_legacy_attrib(AVFormatContext *s, AVIOContext *pb)
         ff_get_guid(pb, &guid);
         type   = avio_rl32(pb);
         length = avio_rl32(pb);
-        if (!length)
+        if (length <= 0)
             break;
         if (ff_guidcmp(&guid, ff_metadata_guid)) {
             av_log(s, AV_LOG_WARNING, "unknown guid "FF_PRI_GUID", expected metadata_guid; "