avcodec/wavpack: Fix integer overflow in wv_unpack_stereo()

Fixes: runtime error: signed integer overflow: 2080374785 + 2080374784 cannot be represented in type 'int'
Fixes: 2351/clusterfuzz-testcase-minimized-5359403240783872

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 73ea2a028e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/wavpack.c

@@ -474,7 +474,7 @@ static inline int wv_unpack_stereo(WavpackFrameContext *s, GetBitContext *gb,
         }
 
         if (type == AV_SAMPLE_FMT_S16P) {
-            if (FFABS(L) + FFABS(R) > (1<<19)) {
+            if (FFABS(L) + (unsigned)FFABS(R) > (1<<19)) {
                 av_log(s->avctx, AV_LOG_ERROR, "sample %d %d too large\n", L, R);
                 return AVERROR_INVALIDDATA;
             }