Fix misuse use of window_gettupleslot function (src/backend/executor/nodeWindowAgg.c)

Hi.

Per Coverity.

CID 1635309: (#1 of 1): Unchecked return value (CHECKED_RETURN)
7. check_return: Calling window_gettupleslot without checking return value
(as is done elsewhere 8 out of 9 times).

The function "window_gettupleslot" can fail.

All other calls check the return, In this case it could not be different.

Fix by checking the return and reporting a message to the user,
in case of failure.
The error message, however, I'm not sure if it's ideal.

best regards,
Ranier Vilela