Reduce lock contention in sp_search_pv()

Reduce lock contention in sp_search_pv()

In less then 1% of cases value > sp->bestValue, so avoid
an useless lock in the common case. This is the same change
already applied to sp_search().

Also SplitPoint futilityValue is not volatile because
never changes after has been assigned in split()

No functional change.

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