LU-13929 lnet: modify assertion in lnet_post_send_locked

LU-13929 lnet: modify assertion in lnet_post_send_locked

Check that the pointer to the local interface is not NULL
before asserting. While checking if local ni is the destination,
the assertion may attempt to dereference pointer to local
interface after it has already been cleaned up on shutdown.

Test-Parameters: trivial
Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Change-Id: I0f4be04a728a7243823bec70f9efbe52bcb104b3
Reviewed-on: https://review.whamcloud.com/40749
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Cyril Bordage <cbordage@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>