pgsql: In rebuild_relation(), don't access an already-closed relcache e

In rebuild_relation(), don't access an already-closed relcache entry.

This reliably fails with -DRELCACHE_FORCE_RELEASE, as reported by
Andrew Dunstan, and could sometimes fail in normal operation, resulting
in a wrong persistence value being used for the transient table.
It's not immediately clear to me what effects that might have beyond
the risk of a crash while accessing OldHeap->rd_rel->relpersistence,
but it's probably not good.

Bug introduced by commit f41872d0c, and made substantially worse by
commit 85b506bbf, which added a second such access significantly
later than the heap_close. I doubt the first reference could fail
in a production scenario, but the second one definitely could.

Discussion: https://postgr.es/m/7b52f900-0579-cda9-ae2e-de5da17090e6@2ndQuadrant.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/dbca84f04ed5debe748029699aa44fa86beca32d

Modified Files
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src/backend/commands/cluster.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)