LU-18751 lnet: Segfault in lnetctl fault command

LU-18751 lnet: Segfault in lnetctl fault command

"lnetctl fault reset 0" and similar variations cause a segfault. This
is caused by a null pointer that is not checked in the code.

Test-Parameters: trivial
Signed-off-by: Frank Sehr <fsehr@whamcloud.com>
Change-Id: Iec580b19f97c2a189ae8f29444bf3e3cc91d78a0
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58216
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Manish Regmi <mregmi@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>