Insteon: Fix Bug in Link Scan Callback When Nack Received

Clearing $current_scan_device was resulting in an undefined object.  This would happen when a device's aldb was listed as out-of-sync.  In that scenario, query_aldb_delta would be called, but it would immediately call the aldb_changed_callback without exiting the loop.
The callback would then set the current_scan_device and call scan_link_table on that device.  At this point, a _send_msg would be queued and the loop would begin to exit. As it exited it would fall all the way back to line 117 of Insteon.pm and clear the current_scan_device.

This patch prevents that error, by not relying on the state of an otherwise global variable in the logic tree.