avcodec/mlpdsp: Fix a invalid shift in ff_mlp_rematrix_channel()

Fixes: left shift of negative value -2
Fixes: 20305/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUEHD_fuzzer-5677196618498048

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Jai Luthra <me@jailuthra.in>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mlpdsp.c

@@ -79,7 +79,7 @@ void ff_mlp_rematrix_channel(int32_t *samples,
 
         if (matrix_noise_shift) {
             index &= access_unit_size_pow2 - 1;
-            accum += noise_buffer[index] << (matrix_noise_shift + 7);
+            accum += noise_buffer[index] * (1 << (matrix_noise_shift + 7));
             index += index2;
         }