Avoid a notice for an undefined $checksums variable when updating from pre-3.7 to post-3.7.

Merges [25915] to the 3.7 branch.

props GaryJ.
fixes #25689.

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git-svn-id: http://core.svn.wordpress.org/branches/3.7@25879 1a063a9b-81f0-0310-95a4-ce76da25c4cd