BRIL unpacker: Follow-up from "Perform BX map decoding on the fly"

The following discussion from !46 (merged) should be addressed:

• @pzejdl started a discussion: (+3 comments)

  Should we also check that wr_ptr doesn't go behind the end of the write buffer? Not sure if it can happen in this case. But can be checked by some assert condition if we pass the end of the buffer to this function.

assigned to @tjames