avcodec/alac: Fix invalid shifts in 20/24 bps

Fixes: left shift of negative value -256
Fixes: 16892/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALAC_fuzzer-4880802642395136

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

libavcodec/alac.c

@@ -397,13 +397,13 @@ static int decode_element(AVCodecContext *avctx, AVFrame *frame, int ch_index,
     case 20: {
         for (ch = 0; ch < channels; ch++) {
             for (i = 0; i < alac->nb_samples; i++)
-                alac->output_samples_buffer[ch][i] <<= 12;
+                alac->output_samples_buffer[ch][i] *= 1 << 12;
         }}
         break;
     case 24: {
         for (ch = 0; ch < channels; ch++) {
             for (i = 0; i < alac->nb_samples; i++)
-                alac->output_samples_buffer[ch][i] <<= 8;
+                alac->output_samples_buffer[ch][i] *= 1 << 8;
         }}
         break;
     }