s3: smbd: Ensure srvstr_pull_req_talloc() always NULLs out *dest.

s3: smbd: Ensure srvstr_pull_req_talloc() always NULLs out *dest.

Robert Morris <rtm@lcs.mit.edu> noticed that in the case
where srvstr_pull_req_talloc() is being called with
buffer remaining == 0, we don't NULL out the destination
pointed which is *always* done in the codepaths inside
pull_string_talloc(). This prevents a crash in the caller.

Remove knownfail.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15420

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>