Prevent customizer header image list from listing user images twice when no theme-specified images exist

fixes #27839. props mcsf

git-svn-id: https://develop.svn.wordpress.org/trunk@28152 602fd350-edb4-49c9-b593-d223f7449a82
WordPress 3.9.


git-svn-id: https://develop.svn.wordpress.org/trunk@28153 602fd350-edb4-49c9-b593-d223f7449a82