pgsql: Fix TwoPhaseGetDummyBackendId().

Fix TwoPhaseGetDummyBackendId().

This was broken in commit ed0b409d22346b1b027a4c2099ca66984d94b6dd,
which revised the GlobalTransactionData struct to not include the
associated PGPROC as its first member, but overlooked one place where
a cast was used in reliance on that equivalence.

The most effective way of fixing this seems to be to create a new function
that looks up the GlobalTransactionData struct given the XID, and make
both TwoPhaseGetDummyBackendId and TwoPhaseGetDummyProc rely on that.

Per report from Robert Ross.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/5cf2307c98f6daeac3d42e74f4e2d46ab3bf0a80

Modified Files
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src/backend/access/transam/twophase.c |   76 ++++++++++++++++++++-------------
1 files changed, 46 insertions(+), 30 deletions(-)