avfilter/vf_elbg: Use unsigned for shifting into the top bit

Fixes: part of CID1355110 Overflowed constant

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 2af95b9214)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/vf_elbg.c

@@ -194,7 +194,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *frame)
         p0 = (uint8_t *)out->data[0];
 
         for (i = 0; i < elbg->codebook_length; i++) {
-            const int al =  elbg->use_alpha ? elbg->codebook[i*4+3] : 0xff;
+            const unsigned al =  elbg->use_alpha ? elbg->codebook[i*4+3] : 0xff;
             pal[i] =  al                    << 24  |
                      (elbg->codebook[i*4+2] << 16) |
                      (elbg->codebook[i*4+1] <<  8) |