Workaround a bug in Position::has_mate_threat()

It seems that pos.has_mate_threat() changes the position !

So that calling MovePicker c'tor before or after the
has_mate_threat() call changes the things !

Bug was unhidden by previous patch that makes MovePicker c'tor
to generate, score and sort good captures under some circumstances.

Because scoring the captures is position dependent it seems that
the moves returned by MovePicker are different when c'tor is
called before has_mate_threat()

Of course this is only a workaround because the real bug is still
hidden :-(

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