[client] x11: fix BadAtom when pasting large buffers

To start a clipboard incr transfer the client has to delete the INCR
window property as the reply to the selection. This deletion generates a
property change event with the type delete, errornously triggering the
incr processing of the data. This patch corrects this by ignoring
property deletions.

client/displayservers/X11/clipboard.c

@@ -76,6 +76,9 @@ bool x11CBEventThread(const XEvent xe)
       return true;
 
     case PropertyNotify:
+      if (xe.xproperty.state != PropertyNewValue)
+        break;
+
       if (xe.xproperty.atom == x11atoms.SEL_DATA)
       {
         if (x11cb.lowerBound == 0)