In some functions dealing with strings and character sets, the wrong
pointers were saved for restoration in THD::rollback_item_tree_changes().
This could potentially cause random corruption or crashes.

Fixed by passing the original Item ** locations, not local stack copies.

Also remove unnecessary use of default arguments.


sql/item.cc:
  Function agg_item_charsets() now handles non-consequtive Item *'s.
sql/item.h:
  Remove use of default argument.
sql/item_cmpfunc.cc:
  Remove use of default argument.
sql/item_func.cc:
  Remove use of default argument.
sql/item_func.h:
  Function agg_item_charsets() now handles non-consequtive Item *'s.
sql/item_strfunc.cc:
  Pass original Item **'s to agg_arg_charsets(), not local copies, to ensure
  proper restoration in THD::rollback_item_tree_changes().
sql/item_sum.cc:
  Remove use of default argument.