Fix for compiler warning triggered in WinGetFuncArgInPartition()

Hello,

This is a minor change that avoids a "maybe uninitialized" warning that
I've been seeing since:

25a30bbd423 Add IGNORE NULLS/RESPECT NULLS option to Window functions.
src/backend/postgres_lib.a.p/executor_nodeWindowAgg.c.o../src/backend/executor/nodeWindowAgg.c:
In function
‘WinGetFuncArgInPartition’:../src/backend/executor/nodeWindowAgg.c:3809:33:
warning:‘notnull_relpos’ may be used uninitialized
[-Wmaybe-uninitialized] 3809 | } while (notnull_offset <
notnull_relpos); |
~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~../src/backend/executor/nodeWindowAgg.c:3723:33:
note: ‘notnull_relpos’was declared here 3723 | int
notnull_relpos; | ^~~~~~~~~~~~~~

To calm the compiler's nerves this patch moves the initialization of
notnull_relpos to later in the function.

best.

-greg