It should always be the case that a question category and its parent
category belong to the same context. However, over the years, we have
observed that this becomes violated by some data in some Moodle sites.
(The suspicion is that this is a backup/restor bug of some sort which
has never been tracked down.)
Since this happens, we should make the code robust to it, which is what
this change does. It ensure that when loading child categories, we only
consider ones in the same context.
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