A fix for Bug#6042 "constants propogation works olny once (prepared
  statements)": reset item->marker in Item::cleanup, as it's used
  in propogate_cond_constants. No test case as the only way I could
  come up with to show the problem is EXPLAIN, and EXPLAIN is painful
  to use in the test suite.


sql/item.h:
  A fix for Bug#6042 "constants propogation works olny once (prepared
  statements)": reset item->marker in Item::cleanup, as it's used
  in propogate_cond_constants