BUG/MEDIUM: lb_fas: Don't test the server's lb_tree from outside the lock

BUG/MEDIUM: lb_fas: Don't test the server's lb_tree from outside the lock

In the function fas_srv_reposition(), the server's lb_tree is tested from
outside the lock. So it is possible to remove it after the test and then call
eb32_insert() in fas_queue_srv() with a NULL root pointer, which is
invalid. Moving the test in the scope of the lock fixes the bug.

This issue was reported on Github, issue #126.

This patch must be backported to 2.0, 1.9 and 1.8.