avcodec/ituh263dec: Fix undefined left shift of negative number

Fixes ticket #8160.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 324487b596)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/ituh263dec.c

@@ -1271,7 +1271,7 @@ int ff_h263_decode_picture_header(MpegEncContext *s)
         for(i=0; i<13; i++){
             for(j=0; j<3; j++){
                 int v= get_bits(&s->gb, 8);
-                v |= get_sbits(&s->gb, 8)<<8;
+                v |= get_sbits(&s->gb, 8) * (1 << 8);
                 av_log(s->avctx, AV_LOG_DEBUG, " %5d", v);
             }
             av_log(s->avctx, AV_LOG_DEBUG, "\n");