Fix temporary memory leak in system table index scans

Fix temporary memory leak in system table index scans

Commit 811af9786b introduced palloc() calls into systable_beginscan()
and systable_beginscan_ordered().  But there was no pfree(), as is the
usual style.

It turns out that an ANALYZE of a partitioned table can invoke many
thousand system table index scans, and this memory is not cleaned up
until the end of the command, so this can temporarily leak quite a bit
of memory.  Maybe there are improvements to be made at a higher level
about this, but for now, insert a couple of corresponding pfree()
calls to fix this particular issue.

Reported-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://www.postgresql.org/message-id/Z0XTfIq5xUtbkiIh@pryzbyj2023