avcodec/wavpack: Fix runtime error: signed integer overflow: 1886191616 + 277872640 cannot be represented in type 'int'

Fixes: 2181/clusterfuzz-testcase-minimized-6314784322486272

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 c996374d4d)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/wavpack.h

@@ -94,7 +94,7 @@ typedef struct Decorr {
 typedef struct WvChannel {
     int median[3];
     int slow_level, error_limit;
-    int bitrate_acc, bitrate_delta;
+    unsigned bitrate_acc, bitrate_delta;
 } WvChannel;
 
 // macros for manipulating median values