Bluetooth: Fix handling of unexpected SMP PDUs

The conn->smp_chan pointer can be NULL if SMP PDUs arrive at unexpected
moments. To avoid NULL pointer dereferences the code should be checking
for this and disconnect if an unexpected SMP PDU arrives. This patch
fixes the issue by adding a check for conn->smp_chan for all other PDUs
except pairing request and security request (which are are the first
PDUs to come to initialize the SMP context).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
CC: stable@vger.kernel.org
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>