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.

Change-Id: Id33ec048f8ecfd7f715feafadfa4ea618cdbac0d
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>