Optimise get_term to not query for term_id = 0. Also improve the prepared...

Optimise get_term to not query for term_id = 0. Also improve the prepared query to use %d for the term_id.
Fixes #18076 props mdawaffe.

git-svn-id: https://core.svn.wordpress.org/trunk@18591 1a063a9b-81f0-0310-95a4-ce76da25c4cd