Add check to make sure a valid argument was passed to `get_page_uri()`.

Props Viper007Bond.
Fixes #24491.



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Only variables should be passed by reference.

Props Askapache.
Fixes #23032.


Built from https://develop.svn.wordpress.org/trunk@25263


git-svn-id: http://core.svn.wordpress.org/trunk@25231 1a063a9b-81f0-0310-95a4-ce76da25c4cd