Check if term exists before trying to use it.

r775010 fixed a bug by looking up the term slug, but introduced a new one by failing to check if the term existed. If the hashtag is being used for the first time, the term won't exist yet and will fail.


git-svn-id: https://plugins.svn.wordpress.org/tagregator/trunk@775588 b8457f37-d9ea-0310-8a92-e5e31aec5664