pgsql: Fix unnecessary use of moving-aggregate mode with non-moving fra

Fix unnecessary use of moving-aggregate mode with non-moving frame.

When a plain aggregate is used as a window function, and the window
frame start is specified as UNBOUNDED PRECEDING, the frame's head
cannot move so we do not need to use moving-aggregate mode. The check
for that was put into initialize_peragg(), failing to notice that
ExecInitWindowAgg() calls that function before it's filled in
winstate->frameOptions. Since makeNode() would have zeroed the field,
this didn't provoke uninitialized-value complaints, nor would the
erroneous decision have resulted in more than a little inefficiency.
Still, it's wrong, so move the initialization of
winstate->frameOptions earlier to make it work properly.

While here, also fix a thinko in a comment. Both errors crept in in
commit a9d9acbf2 which introduced the moving-aggregate mode.

Spotted by Vallimaharajan G. Back-patch to all supported branches.

Discussion: https://postgr.es/m/18e7f2a5167.fe36253866818.977923893562469143@zohocorp.com

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a94f51a7bfc9e65ce3799d8cb12945bd596c5ba8

Modified Files
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src/backend/executor/nodeWindowAgg.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)