Fix subtle race with slave allocation

When allocating a slave we set both is_searching
and splitPoint under lock protection.

Unfortunatly the order in which the variables are
set is not defined. This article was very clarifying:

http://software.intel.com/en-us/blogs/2007/11/30/volatile-almost-useless-for-multi-threaded-programming/

So when in idle loop we test for is_searching and then
access splitPoint, it could happen that splitPoint is still
not updated leading to a possible crash.

Fix the race lock protecting splitPoint access.

No functional change.

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