Initialize $is_404 to true instead of false if we know we are processing a 404
Check for the existence of a valid $post class member in the $wp_query global before accessing it
Check for presence of $post object in $wp_query before referencing it

Conflicts:
	wpsc-components/theme-engine-v1/helpers/template.php
Merge pull request #1534 from JeffPyeBrook/pr-1533-check-post-exists-before-using-on-404

Fix PHP warnings and when processing 404 page for product