Avoid our pre-flight writable checks if get_core_checksums() doesn't exist yet.

Thus, it will not work for 3.6 => 3.7, only 3.7+.

see #22704.


git-svn-id: https://develop.svn.wordpress.org/trunk@25798 602fd350-edb4-49c9-b593-d223f7449a82