Don't overwrite the `length` property unnecessarily when generating metadata for audio and video files.

Props jrf, wonderboymusic.
Fixes #29176.

Built from https://develop.svn.wordpress.org/trunk@29728


git-svn-id: http://core.svn.wordpress.org/trunk@29502 1a063a9b-81f0-0310-95a4-ce76da25c4cd