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 4d355a8336.  Hence, back-patch to all supported versions.
There's no bug in HEAD, though, thanks to our recent tab-completion
rewrite.

src/bin/psql/tab-complete.c

@@ -2919,7 +2919,7 @@ psql_completion(char *text, int start, int end)
 			 pg_strcasecmp(prev_wd, "LABEL") == 0)
 	{
 		static const char *const list_SECURITY_LABEL_preposition[] =
-		{"ON", "FOR"};
+			{"ON", "FOR", NULL};
 
 		COMPLETE_WITH_LIST(list_SECURITY_LABEL_preposition);
 	}