Fix crash when non-creator being an iteration on shared radix tree

Hi,

I found that a server crashes due to a null-pointer-dereference if a
process attached to the shared radix tree begins an iteration on it,
because we don't create the memory context for iter_context at
RT_ATTACH(). There is no code in the core causing this crash in the
core since in parallel vacuum, the leader process always creates the
shared radix tree and begins the iteration. However it could happen in
external extensions. I've attached the patch to fix it and I think it
should be backpatched to v17.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com