Problem: passing a non-constant name to the NAME_CONST function results in a crash.

Fix: check the NAME_CONST name argument; return fake item type if we got
non-constant argument(s).


mysql-test/r/func_misc.result:
  Fix for bug #32559: connection hangs on query with name_const
    - test result.
mysql-test/t/func_misc.test:
  Fix for bug #32559: connection hangs on query with name_const
    - test case.
sql/item.cc:
  Fix for bug #32559: connection hangs on query with name_const
    - Item_name_const::type() now returns NULL_ITEM if non-constant arguments 
      were used to create the item to avoid wrong type casting.
sql/item.h:
  Fix for bug #32559: connection hangs on query with name_const
    - NAME_CONST name argument checked for invariability.