wayland: Avoid using uninitialized memory

_gdk_wayland_cursor_get_buffer was not initializing
its out variables in the 'not found' case. This
was showing up in protocol traces as garbage hotspots
being sent to the compositor.

Closes: https://gitlab.gnome.org/GNOME/gtk/issues/1328

gdk/wayland/gdkcursor-wayland.c

@@ -259,6 +259,14 @@ _gdk_wayland_cursor_get_buffer (GdkCursor *cursor,
       if (wayland_cursor->surface.cairo_surface)
         return _gdk_wayland_shm_surface_get_wl_buffer (wayland_cursor->surface.cairo_surface);
     }
+  else
+    {
+      *hotspot_x = 0;
+      *hotspot_y = 0;
+      *w = 0;
+      *h = 0;
+      *scale = 1;
+    }
 
   return NULL;
 }