Bug#30982 CHAR(..USING..) can return a not-well-formed string
Bug#30986 Character set introducer followed by a HEX string can return bad result
check_well_formed_result moved to Item from Item_str_func
fixed Item_func_char::val_str for proper ucs symbols converting
added check for well formed strings for correct conversion of constants with underscore
charset


mysql-test/r/ctype_ucs.result:
  test result
mysql-test/r/ctype_utf8.result:
  test result
mysql-test/t/ctype_ucs.test:
  test case
mysql-test/t/ctype_utf8.test:
  test case
sql/item.cc:
  check_well_formed_result() moved from Item_str_func
sql/item.h:
  check_well_formed_result() moved from Item_str_func
sql/item_strfunc.cc:
  check_well_formed_result moved to Item
  fixed Item_func_char::val_str for proper ucs symbols converting
sql/item_strfunc.h:
  check_well_formed_result moved to Item
sql/sql_yacc.yy:
  added check for well formed string