avcodec/wnv1: Fix runtime error: left shift of negative value -1

Fixes: 1338/clusterfuzz-testcase-minimized-6485546354343936

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

libavcodec/wnv1.c

@@ -52,7 +52,7 @@ static inline int wnv1_get_code(WNV1Context *w, int base_value)
     if (v == 15)
         return ff_reverse[get_bits(&w->gb, 8 - w->shift)];
     else
-        return base_value + ((v - 7) << w->shift);
+        return base_value + ((v - 7U) << w->shift);
 }
 
 static int decode_frame(AVCodecContext *avctx,