pgsql: 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(at)mail(dot)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.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/2d61d88d8d0be910b54d28448e3fc94f94995792

Modified Files
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src/bin/psql/tab-complete.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)