check_agg_arguments_walker threw an error upon seeing a SRF or window
function, but that is too aggressive: if the function is within a
sub-select then it's perfectly fine.  I broke the SRF case in commit
0436f6bd by copying the logic for window functions ... but that was
broken too, and had been since commit eaccfded.

Repair both cases in HEAD, and the window function case back to 9.3.
9.2 gets this right.