This moves unsafe_block_from_ptr calls outside of the heap lock.

We assume here that concurrent call to another heap function on a block
being freed is undefined, and it should then be safe to do so:

* The block type or base offset never change after a block has been
  allocated and until it is freed.

* Block flags such as BLOCK_FLAG_LARGE, or BLOCK_FLAG_USER_INFO also
  never change after a block has been allocated.

* Other block flags are only read and modified inside the heap lock.