Safe guard some wild and ugly casts

These casts are needed but plain ugly, at least be
sure they don't hide any subtle conversion bug.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/bitbase.cpp

@@ -89,6 +89,7 @@ void generate_kpk_bitbase(uint8_t bitbase[]) {
   int i, j, b;
   for(i = 0; i < 24576; i++) {
     for(b = 0, j = 0; j < 8; b |= (compress_result(Bitbase[8*i+j]) << j), j++);
+    assert(b == int(uint8_t(b)));
     bitbase[i] = (uint8_t)b;
   }