Windows: cm_ForceNewConnections serverp == NULL

If serverp == NULL, return immediately.  Do not dereference a NULL
pointer.

Change-Id: I47781a03f22a83289a23b30ff375249fec18ff51
Reviewed-on: http://gerrit.openafs.org/10845
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>