3.7 regression: Fix the width of captioned images inserted into the visual editor.

Pulls some arithmetic outside of some string concatenation to avoid a nasty uglify.js regression: https://github.com/mishoo/UglifyJS2/pull/330.

props LucP.
fixes #25700 for trunk.


git-svn-id: https://develop.svn.wordpress.org/trunk@25921 602fd350-edb4-49c9-b593-d223f7449a82