swrap: fix invalid read in swrap_sendmsg_unix_scm_rights()

Here the fds_out array is larger than the fds_in array, so we can
only copy the fds_in array using size_fds_in, leaving the last slot
of fds_out untouched, which is filled by fds_out[num_fds_in] = pipefd[0]
later.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>