Path -> Simplify for Path with Powerstroke LPE turns object to path

[Hachmann]

I guess this might be an implication of the new 'path operations try to turn things to path first' change, but I think it shouldn't apply here (but I understand this is disputable, could well be intentional).

This is in 0.92+devel (458da60, 2018-01-08)

In 0.92.2, trying to use Path -> Simplify (or Ctrl+L) on a powerstroked path makes it look strange the after one to three times, and then vanish if applied repeatedly... (something to do with where it ends up, coordinates are very strange).