Add _doing_it_wrong() when calling is_amp_endpoint() before queried object is available

[westonruter]

Ever since 0.4.2, the AMP plugin has issued a _doing_it_wrong() when is_amp_endpoint() is called before the parse_query action. This was needed to ensure that the amp query var was parsed and available for inspection.

It turns out that this did not go far enough. As of 1.0 the is_amp_endpoint() needs to look at the queried object as well to determine whether AMP has been enabled/disabled for a given post on singular queries. The queried object is not available at parse_query since the posts have not been queried yet at this point. Rather, it is only at the wp action that we can be assured that $wp_query->post has been populated and thus that the queried object is present for inspection.

Also make sure that only a WP_Post is passed into AMP_Post_Type_Support::get_support_errors().

Without the changes in this PR, if you have a plugin that does:

add_action( 'parse_query', function() {
	if ( function_exists( 'is_amp_endpoint' ) && is_amp_endpoint() ) {
		add_action( 'wp_footer', function() {
			echo "HELLO PAIRED AMP!";
		} );
		add_action( 'amp_post_template_footer', function() {
			echo "HELLO CLASSIC AMP!";
		} );
	} else {
		add_action( 'wp_footer', function() {
			echo "HELLO NOT AMP!";
		} );
	}
} );

You will get PHP notices when viewing a singular post in paired mode:

Notice: Trying to get property 'post_type' of non-object in /app/public/content/plugins/amp/includes/class-amp-post-type-support.php on line 80
Notice: Trying to get property 'ID' of non-object in /app/public/content/plugins/amp/includes/class-amp-post-type-support.php on line 101

Changing parse_query to wp fixes the issue in plugin code.

Issue discovered originally in a Jetpack PR: Automattic/jetpack#10945 (comment)