Bluetooth: host: Replace length check assert with if statement

A few of the length checks that deal with HCI packets coming from the
controller were using assert statements. But the recommended practice is
to drop invalid packets and continue execution whenever a malformed
packet arrives from an external source, so replace those assert
statements with branches that will drop the packet and return.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>