pgsql: Fix plancache refcount leak after error in ExecuteQuery.

Fix plancache refcount leak after error in ExecuteQuery.

When stuffing a plan from the plancache into a Portal, one is
not supposed to risk throwing an error between GetCachedPlan and
PortalDefineQuery; if that happens, the plan refcount incremented
by GetCachedPlan will be leaked. I managed to break this rule
while refactoring code in 9dbf2b7d7. There is no visible
consequence other than some memory leakage, and since nobody is
very likely to trigger the relevant error conditions many times
in a row, it's not surprising we haven't noticed. Nonetheless,
it's a bug, so rearrange the order of operations to remove the
hazard.

Noted on the way to looking for a better fix for bug #17053.
This mistake is pretty old, so back-patch to all supported
branches.

Branch
------
REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c50596cdc9234b15359761bdeddec5395fbd57c5

Modified Files
--------------
src/backend/commands/prepare.c | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)