Sub-CALLs of procedures zapped thd->current_arena for good.


mysql-test/r/sp.result:
  New test case for BUG#4579.
mysql-test/t/sp.test:
  New test case for BUG#4579.
sql/sp_head.cc:
  Keep and restore old thd->current_arena before/after executing an SP,
  instead of just setting it to 0 afterwards.