s3:smbd: always clear filter_subreq in smb2srv_client_mc_negprot_next()

s3:smbd: always clear filter_subreq in smb2srv_client_mc_negprot_next()

Commit 5d66d5b84f87267243dcd5223210906ce589af91 introduced a
'verify_again:' target, if we ever hit that, we would leak
the existing filter_subreq.

Moving it just above a possible messaging_filtered_read_send()
will allow us to only clear it if we actually create a new
request. That will help us in the next commits.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15346

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