pgsql: Use ereport rather than elog in WinCheckAndInitializeNullTreatme

Use ereport rather than elog in WinCheckAndInitializeNullTreatment.

Previously WinCheckAndInitializeNullTreatment() used elog() to emit an
error message. ereport() should be used instead because it's a
user-facing error. Also use existing get_func_name() to get a
function's name, rather than own implementation.

Moreover add an assertion to validate winobj parameter, just like
other window function API.

Reported-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Author: Tatsuo Ishii <ishii(at)postgresql(dot)org>
Reviewed-by: Chao Li <lic(at)highgo(dot)com>
Discussion: https://postgr.es/m/2952409.1760023154%40sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5f3808646f67260d5dc0d95f2602886da5cf5c52

Modified Files
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src/backend/executor/nodeWindowAgg.c | 20 ++++++++------------
1 file changed, 8 insertions(+), 12 deletions(-)