Windows: Do not reset cm_buf.offset on error

Windows: Do not reset cm_buf.offset on error

When an error occurs the cm_buf_t is not removed from the
hash tables.  Since the scacheHashTable hash is built from
the fid and the offset it is not safe to reset the offset field.
Resetting the offset field results in an assertion failure
during buffer recycling.

Reviewed-on: http://gerrit.openafs.org/8452
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit f56c01d313140b625b4342feddfe464342d1ced4)

Change-Id: I888e56c8c34bcae58c51ba965fd59e5539e1bab5
Reviewed-on: http://gerrit.openafs.org/8647
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>