cli_credentials_parse_string: fix parsing of principals

cli_credentials_parse_string: fix parsing of principals

When parsing a principal-like name, user name was left with full
principal instead of taking only the left part before '@' sign.

>>> from samba import credentials
>>> t = credentials.Credentials()
>>> t.parse_string('admin@realm.test', credentials.SPECIFIED)
>>> t.get_username()
'admin@realm.test'

The issue is that cli_credentials_set_username() does a talloc_strdup()
of the argument, so we need to change order of assignment to allow
talloc_strdup() to copy the right part of the string.

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