exo: Fix crash in SetCursor

SetCursor can crash in rare circumstances when no event has been
processed yet, but SetCursor is called by the client.
Adds line to initialize the cursor widget if it does not exist.

BUG=chromium:648724
TEST=none

Review-Url: https://codereview.chromium.org/2355953003
Cr-Commit-Position: refs/heads/master@{#419947}

components/exo/pointer.cc

@@ -72,6 +72,9 @@ void Pointer::SetCursor(Surface* surface, const gfx::Point& hotspot) {
   if (!focus_)
     return;
 
+  if (!widget_)
+    CreatePointerWidget();
+
   // If surface is different than the current pointer surface then remove the
   // current surface and add the new surface.
   if (surface != surface_) {