avcodec/pictordec: Fix runtime error: left shift of 64 by 25 places cannot be represented in type 'int'

Fixes: 724/clusterfuzz-testcase-6738249571631104

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

@@ -57,7 +57,7 @@ static void picmemset_8bpp(PicContext *s, AVFrame *frame, int value, int run,
     }
 }
 
-static void picmemset(PicContext *s, AVFrame *frame, int value, int run,
+static void picmemset(PicContext *s, AVFrame *frame, unsigned value, int run,
                       int *x, int *y, int *plane, int bits_per_plane)
 {
     uint8_t *d;