avcodec/wavpack: Fix 280:22: runtime error: left shift of negative value -1

Fixes: 653/clusterfuzz-testcase-5773837415219200
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/wavpack.c

@@ -264,7 +264,7 @@ error:
 static inline int wv_get_value_integer(WavpackFrameContext *s, uint32_t *crc,
                                        int S)
 {
-    int bit;
+    unsigned bit;
 
     if (s->extra_bits) {
         S <<= s->extra_bits;