Twenty Seventeen: Hardens the logic for calling featured image in `header.php`

This change uses `get_queried_object_id()` here since the `$post` global may not be set before a call to `the_post()`.

Props westonruter.

Fixes #39302.

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git-svn-id: https://core.svn.wordpress.org/trunk@39564 1a063a9b-81f0-0310-95a4-ce76da25c4cd