Minor OrderedStreamingForkController improvements

Minor OrderedStreamingForkController improvements

Follows-up c94dea7029cae.

* Avoid strlen() where a strict check suffices.
* Use substr() for both comparison and stripping, avoid
  potentially expensive, but also fragile, because strlen()-1
  can  produce $str[-1], which would
  > PHP Notice:  Uninitialized string offset: -1
  Luckily, the implied null would work as expected, given that
  `null !== "\n"` and produces the same result.

Change-Id: If61e5e412aaa2dc7c00c4441b3b7bd5f04160ec8