avcodec/wavpack: Fix runtime error: signed integer overflow: 2013265955 - -134217694 cannot be represented in type 'int'

Fixes: 1922/clusterfuzz-testcase-minimized-5561194112876544

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 a47273c803)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/wavpack.c

@@ -239,7 +239,7 @@ static int wv_get_value(WavpackFrameContext *ctx, GetBitContext *gb,
             if (get_bits_left(gb) <= 0)
                 goto error;
             if (get_bits1(gb)) {
-                add -= (mid - base);
+                add -= (mid - (unsigned)base);
                 base = mid;
             } else
                 add = mid - base - 1;