Handle pasting of empty selection.

Otherwise, pasting the X11 primary selection when empty results an
error and Xlib forcibly exits.

Signed-off-by: Christoph Lohmann <20h@r-36.net>

st.c

@@ -995,6 +995,8 @@ selnotify(XEvent *e) {
 
 	ofs = 0;
 	xsev = (XSelectionEvent *)e;
+	if (xsev->property == None)
+	    return;
 	do {
 		if(XGetWindowProperty(xw.dpy, xw.win, xsev->property, ofs,
 					BUFSIZ/4, False, AnyPropertyType,