avcodec/alsdec: clear last_acf_mantissa

Fixes: use-of-uninitialized-value
Fixes: 70869/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALS_fuzzer-5476567461986304

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

libavcodec/alsdec.c

@@ -2112,7 +2112,7 @@ static av_cold int decode_init(AVCodecContext *avctx)
         ctx->acf               = av_malloc_array(channels, sizeof(*ctx->acf));
         ctx->shift_value       = av_calloc(channels, sizeof(*ctx->shift_value));
         ctx->last_shift_value  = av_calloc(channels, sizeof(*ctx->last_shift_value));
-        ctx->last_acf_mantissa = av_malloc_array(channels, sizeof(*ctx->last_acf_mantissa));
+        ctx->last_acf_mantissa = av_calloc(channels, sizeof(*ctx->last_acf_mantissa));
         ctx->raw_mantissa      = av_calloc(channels, sizeof(*ctx->raw_mantissa));
 
         ctx->larray = av_malloc_array(ctx->cur_frame_length * 4, sizeof(*ctx->larray));