avformat/webpenc: Check filesize in trailer

not sure this is possible

Fixes: CID1604446 Overflowed constant

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 7734c583f7)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/webpenc.c

@@ -190,7 +190,7 @@ static int webp_write_trailer(AVFormatContext *s)
 
         if (!ret) {
             filesize = avio_tell(s->pb);
-            if (avio_seek(s->pb, 4, SEEK_SET) == 4) {
+            if (filesize >= 8 && avio_seek(s->pb, 4, SEEK_SET) == 4) {
                 avio_wl32(s->pb, filesize - 8);
                 // Note: without the following, avio only writes 8 bytes to the file.
                 avio_seek(s->pb, filesize, SEEK_SET);