Bluetooth: SMP: Fix pairing using debug keys

Fix issue when checking if SMP pairing procedure is allowed to use the
SMP debug keys. This check did not consider the case where the keys
pointer was assigned, but did not contain a valid LTK key.
This resulted in being unable to pair with debug keys without an
existing bond.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>