Add missing NULL terminator to list_SECURITY_LABEL_preposition[].

Add missing NULL terminator to list_SECURITY_LABEL_preposition[].

On the machines I tried this on, pressing TAB after SECURITY LABEL led to
being offered ON and FOR as intended, plus random other keywords (varying
across machines).  But if you were a bit more unlucky you'd get a crash,
as reported by nummervet@mail.ru in bug #14019.

Seems to have been an aboriginal error in the SECURITY LABEL patch,
commit 4d355a8336e0f226.  Hence, back-patch to all supported versions.
There's no bug in HEAD, though, thanks to our recent tab-completion
rewrite.