GIMP stopped trying to read the XCF as soon as an invalid parasite was
encountered. However, in this specific case only the parasite data is
invalid, while the rest of the image is not corrupt.

Instead of terminating when we see a corrupt parasite, we skip to the
offset after the parasite. This may still be corrupt, but we can handle
that correctly, see e.g. the XCF in bugzilla issue 685086, which was
the reason of some of the previous changes.

Additionally:
- We add some logging to make it easier to handle future issues in this
  area.
- We add tests for a NULL parasite name, and for reading a different
  amount of parasite data than we expected. In both cases we return
  NULL instead of a parasite.

(cherry picked from commit 5b39bc96)