avcodec/mpc8: Fix 32bit mask/enum

Fixes: left shift of 1 by 31 places cannot be represented in type 'int'
Fixes: 15817/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPC8_fuzzer-5636626409062400

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit e8bb949ade)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpc8.c

@@ -62,7 +62,7 @@ static inline int mpc8_dec_enum(GetBitContext *gb, int k, int n)
     do {
         n--;
         if (code >= C[n]) {
-            bits |= 1 << n;
+            bits |= 1U << n;
             code -= C[n];
             C -= 32;
             k--;