staging: lustre: fix crash due to NULL networks string

If there is an invalid networks or ip2nets lnet_parse_networks()
gets called with a NULL 'network' string parameter

lnet_parse_networks() needs to sanitize its input string now that
it's being called from multiple places.  Instead, check for
a NULL string everytime the function is called, which reduces the
probability of errors with other code modifications.

Signed-off-by: Amir Shehata <amir.shehata@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5540
Reviewed-on: http://review.whamcloud.com/11626


Reviewed-by: Isaac Huang <he.huang@intel.com>
Reviewed-by: Doug Oucharek <doug.s.oucharek@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>