crimson/os: do not return a value for non-reachable branch

crimson/os: do not return a value for non-reachable branch

for silencing a warning like

btree/btree_lba_manager.cc:177:12: warning: binding dereferenced null pointer to reference has undefined behavior [-Wnull-dereference]
    return *static_cast<btree_range_pin_t*>(nullptr);
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

ceph_abort_msg() is marked [[noreturn]], so we can just use it here
without worrying about complains from compiler.

Signed-off-by: Kefu Chai <kchai@redhat.com>