Fix a (theoretical) race leading to a crash

Fix a (theoretical) race leading to a crash

After we release the SplitPoint lock the master, suppose
is main thread, can safely return and if a "quit" command
is pending, main thread exits and associated Thread object
is freed. So when we access master->is_searching a crash
occurs.

I have never found such a race that is of course very rare
becuase assumes that from lock releasing we go to sleep for
a time long enough for the main thread to end the search and
return. But you can never know, and anyhow a race is a race.

No functional change.

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