Verify vector clock in Valgrind

Reported:
  https://bugs.kde.org/show_bug.cgi?id=341258

Subsequent CHANGELOG in Valgrind (thanks to Bart Van Assche for fixing this):
  https://github.com/svn2github/valgrind/commit/7548e7d2a65e17cc3b75cbab006682984c5487b4

We should repeat the verification on the updated version above.