redirect_canonical returns too early when redirect_url cannot be determined

[wnorris]

redirect_canoncial() returns if the value of $redirect_url is false, before calling the 'redirect_canonical' filter. This prevents plugins from being able to provide a proper redirect URL in the event that WordPress is unable to. The second check AFTER the hook is called should continue to check if $redirect_url is false, but the one BEFORE the hook should not.

[markjaquith]

(In [10448]) Allow plugins to provide a canonical redirect_url even if WordPress does not provide its own. props wnorris. fixes #8975 for trunk