Confirm delete_site_option() exists before calling it during an update.

This function won't yet exist when updating from 2.7 [sic].

Merges [25893] to the 3.7 branch.
fixes #25682.


git-svn-id: https://develop.svn.wordpress.org/branches/3.7@25894 602fd350-edb4-49c9-b593-d223f7449a82