net: bt: Fix setting ll addresses to possible invalid pointers

net_nbuf_ll_src(buf)->addr and net_nbuf_ll_dst(buf)->addr should be
pointing to ctxt area not actual net_nbuf_ll region since the payload
over Bluetooth does not carry any ll addresses.

Change-Id: I87828d74abf2402fdf2a5e34aa8db93aa7c50d08
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>