When picking a "high utility centroid" do not pick one

that has no corresponding points. Not only it is the 
worst possible pick, but also the code was written 
without this case in mind.


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@14341 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavcodec/elbg.c

@@ -105,9 +105,12 @@ static int get_high_utility_cell(elbg_data *elbg)
 {
     int i=0;
     /* Using linear search, do binary if it ever turns to be speed critical */
-    int r = av_random(elbg->rand_state)%elbg->utility_inc[elbg->numCB-1];
+    int r = av_random(elbg->rand_state)%(elbg->utility_inc[elbg->numCB-1]-1) + 1;
     while (elbg->utility_inc[i] < r)
         i++;
+
+    assert(!elbg->cells[i]);
+
     return i;
 }