shell-recorder: Avoid a crash when the cursor texture is NULL

When running as a wayland compositor, MetaCursorTracker might
legitimately not have a displayed cursor because e.g. a client unsets
the cursor surface.

Note that, under an X session, the assumption that the tracker always
has a cursor texture is true because XFixesGetCursorImage() always
returns data even if the cursor isn't visible.

https://bugzilla.gnome.org/show_bug.cgi?id=767001

src/shell-recorder.c

@@ -321,6 +321,9 @@ recorder_fetch_cursor_image (ShellRecorder *recorder)
   guint8 *data;
 
   texture = meta_cursor_tracker_get_sprite (recorder->cursor_tracker);
+  if (!texture)
+    return;
+
   width = cogl_texture_get_width (texture);
   height = cogl_texture_get_height (texture);
   stride = 4 * width;