Windows: cm_ForceNewConnections serverp == NULL

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

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>
(cherry picked from commit 8803c35994eb48605e26831296bd1c57f0adf50a)

Change-Id: I0700b5e55d07f316b79308fae13ca4aa1aba6801
Reviewed-on: http://gerrit.openafs.org/10856
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>