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

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