elbg: fix assert

It seems the condition was flipped from what was intended
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/elbg.c

@@ -111,7 +111,7 @@ static int get_high_utility_cell(elbg_data *elbg)
     while (elbg->utility_inc[i] < r)
         i++;
 
-    assert(!elbg->cells[i]);
+    assert(elbg->cells[i]);
 
     return i;
 }