Bluetooth: Fix clearing sent_cmd pointer when allocating event buffer

We should clear the bt_dev.sent_cmd pointer after using it to allocate a
new HCI event buffer in the bt_buf_get_cmd_complete() function.
Otherwise, there is a risk of reusing the same stored net_buf for
multiple consecutive HCI events in case the controller sents duplicate
or invalid event packets.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>