Problem:

In the function dict_foreign_remove_from_cache(), the rb tree was updated
without actually verifying whether the given foreign key object is there in the
rb tree or not.  There can be an existing foreign key object with the same id 
in the rb tree, which must not be removed.  Such a scenario comes when an
attempt is made to add a foreign key object with a duplicate identifier.

Solution:

When the foreign key object is removed from the dictionary cache, ensure
that the foreign key object removed from the rbt is the correct one.

rb#7168 approved by Jimmy and Marko.