Fix debug code that is built only in Release builds (instead of in Debug builds)

[dsiganos]

In functions:

• ledger_processor::receive_block
• ledger_processor::open_block

the debug code is inside an ifdef NDEBUG block but it should be ifndef NDEBUG.

Also, scan the rest of the code for similar problems. They should be easy to spot.

Raising this as an issue because if I fix right now, we will get conflicts with ongoing PR merging. (https://github.com/nanocurrency/nano-node/pull/4460)

[clemahieu]

This related build error slipped in https://github.com/nanocurrency/nano-node/pull/4487.