rxvt crashes with rxvt: "XError: Request: 18 . 0, Error: 5" when receiving SelectionRequest from window
that leaves property parameter empty.
ICCCM says (http://tronche.com/gui/x/icccm/sec-2.html#s-2.2):
If the specified property is None , the requestor is an obsolete client. Owners are encouraged to support these clients by using the specified target atom as the property name to be used for the reply.
rxvt_selection_send does not check for an empty rq->property and goes ahead with sending XChangeProperty which crashes.
Probably a simple
if (rq->property == 0)
rq->property = rq->target
is sufficient to support such clients.