LU-5540 lnet: fix crash due to NULL networks string

LU-5540 lnet: 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>
Change-Id: Ifcc1f6f74a3e0e804cb65e3d1b83f85a24f44d9b
Reviewed-on: http://review.whamcloud.com/11626
Tested-by: Jenkins
Reviewed-by: Isaac Huang <he.huang@intel.com>
Reviewed-by: Doug Oucharek <doug.s.oucharek@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>