libavcodec/wmalosslessdec: prevent sum of positive numbers from becoming negative

Fixes: left shift of negative value -8321365
Fixes: 20506/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMALOSSLESS_fuzzer-4798062906310656

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

libavcodec/wmalosslessdec.c

@@ -164,7 +164,7 @@ typedef struct WmallDecodeCtx {
     int transient_pos[WMALL_MAX_CHANNELS];
     int seekable_tile;
 
-    int ave_sum[WMALL_MAX_CHANNELS];
+    unsigned ave_sum[WMALL_MAX_CHANNELS];
 
     int channel_residues[WMALL_MAX_CHANNELS][WMALL_BLOCK_MAX_SIZE];