Little fixes

This won't work in C++. `static_cast` down the class inheritance tree is not a safe operation, it will not fail if `a_BlockEntity` is not a `cMobEntity`.
You *could* use the RTTI, but we avoid it as much as we can and we've been successful so far.
Our preferred solution is to use the `a_BlockEntity->GetBlockType()` function to see if it's the correct block type.