Windows: cm_GetNewSCache must return NULL on failure

Windows: cm_GetNewSCache must return NULL on failure

cm_GetNewSCache was leaking a valid cm_scache_t pointer in some
failure cases.  On failure, explicitly set the return value to
NULL.

Change-Id: I074b278f969224aa535abe256ac33a90d0f4e62d
Reviewed-on: http://gerrit.openafs.org/7037
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>