avcodec/alac: Check decorr_shift to avoid invalid shift

Later the decorrelate_stereo call is guarded by channels == 2
and non-zero decorr_left_weight. Make sure decorr_shift is in
the expected shift range for that case.

Fixes: shift exponent 128 is too large for 32-bit type 'int'
Fixes: 23860/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALAC_fuzzer-5751138914402304

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Alexander Strasser <eclipse7@gmx.net>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/alac.c

@@ -302,6 +302,9 @@ static int decode_element(AVCodecContext *avctx, AVFrame *frame, int ch_index,
         decorr_shift       = get_bits(&alac->gb, 8);
         decorr_left_weight = get_bits(&alac->gb, 8);
 
+        if (channels == 2 && decorr_left_weight && decorr_shift > 31)
+            return AVERROR_INVALIDDATA;
+
         for (ch = 0; ch < channels; ch++) {
             prediction_type[ch]   = get_bits(&alac->gb, 4);
             lpc_quant[ch]         = get_bits(&alac->gb, 4);