Fix a crash (assert) when client set serial version < 24 in INIT

When SER_FMT_VER_LOWEST is set to zero, then the test is stupid in INIT because all client works. In mapblock we check if client's serialization version is < 24, but if client sent serialization version < 24 (15 for example) the server set it and tried to send nodes, then BOOM

To resolve the problem:
* Create a different CLIENT_MIN_VERSION to handle this problem
* Remove the exception
* Use an assert in case of bad developer code