fixed apparent typo in opt_sum.cc
I will push this Changeset although it fails select test - failing select 
is better than the code that does not compile.


mysql-test/r/rpl000001.result:
  updated result
mysys/mf_iocache2.c:
  fixed wrong DBUG_ASSERT()
sql/opt_sum.cc:
  fixed an apparent typo to make it compile