Embeds: Avoid "Division by zero" warning in `embed-content.php` template if a...

Embeds: Avoid "Division by zero" warning in `embed-content.php` template if a thumbnail has a zero height.

Props stevenlinx, aegis123.
Fixes #40931.
Built from https://develop.svn.wordpress.org/trunk@41902


git-svn-id: https://core.svn.wordpress.org/trunk@41736 1a063a9b-81f0-0310-95a4-ce76da25c4cd