Fix Coverity issues reported in commit 25a30bbd423.

Fix Coverity issues reported in commit 25a30bbd423.

Fix several issues pointed out by Coverity (reported by Tome Lane).

- In row_is_in_frame(), return value of window_gettupleslot() was not
  checked.

- WinGetFuncArgInPartition() tried to derefference "isout" pointer
  even if it could be NULL in some places.

Besides the issues, I also fixed a compiler warning reported by Álvaro
Herrera.

Moreover, in WinGetFuncArgInPartition refactor the do...while loop so
that the codes inside the loop simpler. Also simplify the case when
abs_pos < 0.

Author: Tatsuo Ishii <ishii@postgresql.org>
Reviewed-by: Paul Ramsey <pramsey@cleverelephant.ca>
Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Reported-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/1686755.1759679957%40sss.pgh.pa.us
Discussion: https://postgr.es/m/202510051612.gw67jlc2iqpw%40alvherre.pgsql