librpc: Do not access name[-1] trying to push "" into a dnsp_name

librpc: Do not access name[-1] trying to push "" into a dnsp_name

This simply matches the behaviour from before e7b1acaddf2ccc7de0301cc67f72187ab450e7b5
when the logic for a trailing . was added.  This matches what is added in
the dnsRecord attribute for a name of "." over the dnsserver RPC
management interface and is based on what Windows does for that name
in (eg) an MX record.

No a security bug because we use talloc and so name will be just the
end of the talloc header.

Credit to OSS-Fuzz

Found using the fuzz_ndr_X fuzzer

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Fri Dec 20 11:33:52 UTC 2019 on sn-devel-184