LU-7734 lnet: fix NULL access in lnet_peer_aliveness_enabled

LU-7734 lnet: fix NULL access in lnet_peer_aliveness_enabled

When a peer is not on a local network, lpni->lpni_net is NULL.
The lpni_net is access in lnet_peer_aliveness_enabled() without
checking if it's NULL. Fixed.

Test-Parameters: trivial
Signed-off-by: Amir Shehata <amir.shehata@intel.com>
Change-Id: If328728e2bda2a19b273140a20c04b22bdda6bc4
Reviewed-on: http://review.whamcloud.com/22183
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Olaf Weber <olaf@sgi.com>