Fix logic inversion in r15626. see #14892. Also note that r15626 fixed a...

Fix logic inversion in r15626. see #14892. Also note that r15626 fixed a s/is_category/is_tag/ typo introduced in r6011, on line 1340.

git-svn-id: https://core.svn.wordpress.org/trunk@15627 1a063a9b-81f0-0310-95a4-ce76da25c4cd