When creating a post containing emoji, the post can be saved incorrectly if MySQL isn't using a character set that supports emoji.

This change prevents the save from occurring, so it can be handled correctly in the UI.

See #21212.

Built from https://develop.svn.wordpress.org/trunk@30345


git-svn-id: http://core.svn.wordpress.org/trunk@30344 1a063a9b-81f0-0310-95a4-ce76da25c4cd