mysql-test/r/func_str.result:
  Add result for bug#17047: CHAR() and IN() can return NULL without
  signaling NULL result.
mysql-test/t/func_str.test:
  Add test case for bug#17047: CHAR() and IN() can return NULL without
  signaling NULL result.
sql/item_strfunc.cc:
  Add Item_str_func::fix_fields() implementation, and set maybe_null to
  TRUE if we are in the SQL mode that requires some functions to return
  null even if they normally do not.
sql/item_strfunc.h:
  Add declaration of Item_str_func::fix_fields().
  Do not reset maybe_null in Item_func_char::fix_length_and_dec().