Bluetooth: Fix possible NULL pointer dereference in cmd_complete

It is now possible to create command complete event without specific
reply data by passing NULL as reply with len 0. Check pointer before
calling memcpy to avoid undefined behaviour.

Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>