avcodec/mimic: Fix runtime error: left shift of negative value -1

Fixes: 1365/clusterfuzz-testcase-minimized-5624158450876416

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mimic.c

@@ -262,7 +262,7 @@ static int vlc_decode_block(MimicContext *ctx, int num_coeffs, int qscale)
 
         coeff = vlcdec_lookup[num_bits][value];
         if (pos < 3)
-            coeff <<= 4;
+            coeff *= 16;
         else /* TODO Use >> 10 instead of / 1001 */
             coeff = (coeff * qscale) / 1001;