Issue #1707928 by joelpittet, jstoller, Polonium: PHP strict warning:…

… Only variables should be passed by reference in .../modules/book/book.module on line 559

modules/book/book.module

@@ -556,7 +556,8 @@ function book_prev($book_link) {
     // The previous page in the book may be a child of the previous visible link.
     if ($prev['depth'] == $book_link['depth'] && $prev['has_children']) {
       // The subtree will have only one link at the top level - get its data.
-      $data = array_shift(book_menu_subtree_data($prev));
+      $subtree = book_menu_subtree_data($prev);
+      $data = array_shift($subtree);
       // The link of interest is the last child - iterate to find the deepest one.
       while ($data['below']) {
         $data = end($data['below']);