avcodec/wavpack: Fix overflow in adding tail

Fixes: signed integer overflow: 2146907204 + 26846088 cannot be represented in type 'int'
Fixes: 8105/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WAVPACK_fuzzer-6233036682166272

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit d13379fb79)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/wavpack.c

@@ -85,7 +85,7 @@ typedef struct WavpackContext {
 
 #define LEVEL_DECAY(a)  (((a) + 0x80) >> 8)
 
-static av_always_inline int get_tail(GetBitContext *gb, int k)
+static av_always_inline unsigned get_tail(GetBitContext *gb, int k)
 {
     int p, e, res;