Use ereport rather than elog in WinCheckAndInitializeNullTreatment.

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@sss.pgh.pa.us>
Author: Tatsuo Ishii <ishii@postgresql.org>
Reviewed-by: Chao Li <lic@highgo.com>
Discussion: https://postgr.es/m/2952409.1760023154%40sss.pgh.pa.us