Bluetooth: L2CAP: Fix null-ptr-deref in l2cap_sock_state_change_cb()

Add the same NULL guard already present in
l2cap_sock_resume_cb() and l2cap_sock_ready_cb().

Fixes: 89bc500e41fc ("Bluetooth: Add state tracking to struct l2cap_chan")
Cc: stable@kernel.org
Signed-off-by: Siwei Zhang <oss@fourdim.xyz>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>