avcodec/ituh263dec: Check for the bitstream end in ff_h263_decode_mb()

Fixes invalid shift

Fixes: 670/clusterfuzz-testcase-4852021066727424

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegReviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ituh263dec.c

@@ -962,6 +962,9 @@ intra:
     }
 end:
 
+    if (get_bits_left(&s->gb) < 0)
+        return AVERROR_INVALIDDATA;
+
         /* per-MB end of slice check */
     {
         int v= show_bits(&s->gb, 16);