avcodec/ituh263dec: Fix runtime error: left shift of negative value -22

Fixes: 639/clusterfuzz-testcase-5143866241974272

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 631f748491)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ituh263dec.c

@@ -524,7 +524,7 @@ retry:
                     }else{
                         level = SHOW_UBITS(re, &s->gb, 5);
                         SKIP_CACHE(re, &s->gb, 5);
-                        level |= SHOW_SBITS(re, &s->gb, 6)<<5;
+                        level |= SHOW_SBITS(re, &s->gb, 6) * (1<<5);
                         SKIP_COUNTER(re, &s->gb, 5 + 6);
                     }
                 }