x11/clutter-device-manager-xi2: Added checking for device and source_device are NULL

Added checking for device and source_device becomes NULL under the case XI_Enter|XI_Leave. We have encountered instances in which the source_device becomes NULL and as a consequence the call to _clutter_input_device_reset_scroll_info() causes a crash. Please verify the fix and let me know if I can combine the two 'if(source_device)' conditions into one. I think those calls are supposed to be in that sequence?

Signed-off-by: Jos Collin jcollin@redhat.com