Add `defensive_assert!` macro (#13423)

* Add defensive_assert macro

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Apply review suggestions

Co-authored-by: Bastian Köcher <git@kchr.de>

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>