LU-12678 lnet: lnet_startup_lndnet: avoid use-after-free

LU-12678 lnet: lnet_startup_lndnet: avoid use-after-free

If lnet_startup_lndni() fails it will free 'ni' (via lnet_ni_free()).
So we mustn't de-reference it in the LASSERT() in that case

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I01e35013e028a8f95f169e25aeb0c344b2310380
Reviewed-on: https://review.whamcloud.com/36855
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>