avcodec/alacdsp: Fix invalid shift in append_extra_bits()

Fixes: left shift of negative value -1
Fixes: 21390/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALAC_fuzzer-6242539519868928

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

libavcodec/alacdsp.c

@@ -49,7 +49,7 @@ static void append_extra_bits(int32_t *buffer[2], int32_t *extra_bits_buffer[2],
 
     for (ch = 0; ch < channels; ch++)
         for (i = 0; i < nb_samples; i++)
-            buffer[ch][i] = (buffer[ch][i] << extra_bits) | extra_bits_buffer[ch][i];
+            buffer[ch][i] = ((unsigned)buffer[ch][i] << extra_bits) | extra_bits_buffer[ch][i];
 }
 
 av_cold void ff_alacdsp_init(ALACDSPContext *c)