pgsql: Fix dangling smgr_owner pointer when a fake relcache entry is fr

Fix dangling smgr_owner pointer when a fake relcache entry is freed.

A fake relcache entry can "own" a SmgrRelation object, like a regular
relcache entry. But when it was free'd, the owner field in SmgrRelation
was not cleared, so it was left pointing to free'd memory.

Amazingly this apparently hasn't caused crashes in practice, or we would've
heard about it earlier. Andres found this with Valgrind.

Report and fix by Andres Freund, with minor modifications by me. Backpatch
to all supported versions.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/f650387b5038b50407d33d1e9b7590483bb11a72

Modified Files
--------------
src/backend/access/transam/xlogutils.c | 3 +++
src/backend/storage/smgr/smgr.c | 42 +++++++++++++++++++++++++++++---
src/include/storage/smgr.h | 1 +
3 files changed, 42 insertions(+), 4 deletions(-)