Fixed the pre-caching of functions. It now gives the expected stack overrun
  error for functions recursing too deep.


mysql-test/r/sp.result:
  New test case for BUG#6022.
mysql-test/t/sp.test:
  New test case for BUG#6022.
sql/sp.cc:
  Cache function first, then recurse, or the pre-caching loops infinitely
  for recursive functions.