[legacy] Fix ZSTDv0*_decodeSequence()

[legacy] Fix ZSTDv0*_decodeSequence()

* Version <= 0.5 could read beyond the end of `dumps`, which points into
  the input buffer.
* Check the validity of `dumps` before using it, if it is out of bounds
  return garbage values. There is no return code for this function.
* Introduce `MEM_readLE24()` for simplicity, since I don't want to trust
  that there is an extra byte after `dumps`.