glx: Ignore DRI2 event for drawables we've destroyed

Since we now actually destroy GLX drawables, we get into situations where
we get events for drawables that no longer exist.  Just ignore the
event in that case.

src/glx/dri2.c

@@ -103,7 +103,7 @@ DRI2WireToEvent(Display *dpy, XEvent *event, xEvent *wire)
 
       /* Ignore swap events if we're not looking for them */
       pdraw = dri2GetGlxDrawableFromXDrawableId(dpy, awire->drawable);
-      if (!(pdraw->eventMask & GLX_BUFFER_SWAP_COMPLETE_INTEL_MASK))
+      if (!pdraw || !(pdraw->eventMask & GLX_BUFFER_SWAP_COMPLETE_INTEL_MASK))
 	 return False;
 
       aevent->serial = _XSetLastRequestRead(dpy, (xGenericReply *) wire);