avformat/sauce: Check avio_size() for failure

Fixes: CID1604592 Overflowed constant

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 759aae590c)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/sauce.c

@@ -34,7 +34,12 @@ int ff_sauce_read(AVFormatContext *avctx, uint64_t *fsize, int *got_width, int g
     AVIOContext *pb = avctx->pb;
     char buf[36];
     int datatype, filetype, t1, t2, nb_comments;
-    uint64_t start_pos = avio_size(pb) - 128;
+    int64_t start_pos = avio_size(pb);
+
+    if (start_pos < 128)
+        return AVERROR_INVALIDDATA;
+
+    start_pos -= 128;
 
     avio_seek(pb, start_pos, SEEK_SET);
     if (avio_read(pb, buf, 7) != 7)