avcodec/pictordec: runtime error: left shift of 15 by 28 places cannot be represented in type 'int'

Fixes: 898/clusterfuzz-testcase-6149765467209728

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

libavcodec/pictordec.c

@@ -62,7 +62,7 @@ static void picmemset(PicContext *s, AVFrame *frame, unsigned value, int run,
 {
     uint8_t *d;
     int shift = *plane * bits_per_plane;
-    unsigned mask  = ((1 << bits_per_plane) - 1) << shift;
+    unsigned mask  = ((1U << bits_per_plane) - 1) << shift;
     value   <<= shift;
 
     while (run > 0) {