Hardcode depth limit for selective search

Because futility margins array has a fixed size we cannot
arbitrarly choose or change the SelectiveDepth parameter,
otherwise we have a crash for values bigger then array size.

On the other hand tweaking of this parameter requires some
modification to the hardcoded margins, so makes sense to hard
code also this very bounded one.

Who wants to experiment is of course free to change the sources.

No functional change.

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