Handle multiple deserialization of a block's entities

Handle multiple deserialization of a block's entities

This fix consists of two parts:
- Clear the list of stored entities. This has no side-effects.
- Catch the case where active entities exist and print a message.
  Clearing the active entitiy list has side-effects that should
  be handled. (those entities are known to the environment and
  to clients).

  As avoiding those side-effects is more complex, and as this
  problem is not expected to occur (with PR #4847 merged), there
  is no real incentive to implement this ATM.

This issue was a contributing factor to bug #4217. With the other
contributing factor removed (PR #4847), this commit makes sure this
factor does not go unnoticed if it ever happens again.