avcodec/alac: Check lpc_quant

lpc_quant of 0 produces undefined behavior, thus disallow this.
If valid samples use this then such a sample would be quite
usefull to confirm the correct&lossles handling of this.

Fixes: libavcodec/alac.c:218:25: runtime error: shift exponent -1 is negative
Fixes: 15273/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALAC_fuzzer-5656388535058432
Fixes: 15276/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALAC_fuzzer-5761238417539072
Fixes: 15315/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALAC_fuzzer-5767260766994432

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

libavcodec/alac.c

@@ -306,7 +306,7 @@ static int decode_element(AVCodecContext *avctx, AVFrame *frame, int ch_index,
             rice_history_mult[ch] = get_bits(&alac->gb, 3);
             lpc_order[ch]         = get_bits(&alac->gb, 5);
 
-            if (lpc_order[ch] >= alac->max_samples_per_frame)
+            if (lpc_order[ch] >= alac->max_samples_per_frame || !lpc_quant[ch])
                 return AVERROR_INVALIDDATA;
 
             /* read the predictor table */