Fix Coverity issue reported in commit 2273fa32bce.

Fix Coverity issue reported in commit 2273fa32bce.

Coverity complains that the return value from gettuple_eval_partition
(stored in variable "datum") in a do..while loop in
WinGetFuncArgInPartition is overwritten when exiting the while
loop. This commit tries to fix the issue by changing the
gettuple_eval_partition call to:

(void) gettuple_eval_partition()

explicitly stating that we discard the return value. We are just
interested in whether we are inside or outside of partition, NULL or
NOT NULL here.

Also enhance some comments for easier code reading.

Reported-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/aPCOabSE4VfJLaky%40paquier.xyz