pgsql: Ensure mark_dummy_rel doesn't create dangling pointers in RelOpt

Ensure mark_dummy_rel doesn't create dangling pointers in RelOptInfos.

When we are doing GEQO join planning, the current memory context is a
short-lived context that will be reset at the end of geqo_eval(). However,
the RelOptInfos for base relations are set up before that and then re-used
across many GEQO cycles. Hence, any code that modifies a baserel during
join planning has to be careful not to put pointers to the short-lived
context into the baserel struct. mark_dummy_rel got this wrong, leading to
easy-to-reproduce-once-you-know-how crashes in 8.4, as reported off-list by
Leo Carson of SDSC. Some improvements made in 9.0 make it difficult to
demonstrate the crash in 9.0 or HEAD; but there's no doubt that there's
still a risk factor here, so patch all branches that have the function.
(Note: 8.3 has a similar function, but it's only applied to joinrels and
thus is not a hazard.)

Branch
------
REL8_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/1de8584fb1b71c98138b1f23808a4f01ab7566cd

Modified Files
--------------
src/backend/optimizer/path/joinrels.c | 26 +++++++++++++++++++++++++-
1 files changed, 25 insertions(+), 1 deletions(-)