Bluetooth: msft: fix null pointer deref on msft_monitor_device_evt

msft_find_handle_data returns NULL if it can't find the handle.
Therefore, handle_data must be checked, otherwise a null pointer
is dereferenced.

Signed-off-by: Soenke Huster <soenke.huster@eknoes.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>