wp_get_single_post and wp_update_post assume post properties

[filosofo]

wp_update_post and wp_get_single_post assume both that the post object they try to retrieve exists and that it has certain properties.

Theses assumptions do not always hold true, as sometimes, for example one can pass arguments to wp_update_post to create a new post object.

Patch does sanity checks for post object properties.

[ryan]

(In [15405]) Sanity check properties in wp_get_single_post(). Props filosofo. fixes #13904 for 3.1

[ryan]

(In [15406]) Sanity check properties in wp_get_single_post(). Props filosofo. fixes #13904 for 3.0.1