radixtree: Fix crash when non-creator begins iteration over shared tree.

radixtree: Fix crash when non-creator begins iteration over shared tree.

Previously, if a backend that attached to a shared tree attempted to
start iteration, it resulted in a crash. This commit resolves the
issue by ensuring iter_context is created in RT_ATTACH().

This fix applies only to v17, where radixtree.h was introduced. In the
master branch, this issue was separately resolved by 960013f2a1, which
eliminated the iter_context entirely.

Reviewed-by: John Naylor
Discussion: https://postgr.es/m/CAD21AoBB2U47V=F+wQRB1bERov_of5=BOZGaybjaV8FLQyqG3Q@mail.gmail.com