Windows: NPGetConnectionCommon buffer too small

Windows: NPGetConnectionCommon buffer too small

If the specified buffer size is too small, say so.  Do not
attempt to copy data in that won't fit.  Do not walk beyond
the end of the allocated memory.

Change-Id: Id4a75273d8ec9e9cc8471a963bc32f6cad59163e
Reviewed-on: http://gerrit.openafs.org/6140
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>