Problem: wrong character set pointer was passed to my_strtoll10_mb2,
which led to DBUG_ASSERT failure in some cases.

  @ mysql-test/r/func_encrypt_ucs2.result
  @ mysql-test/t/func_encrypt_ucs2.test
  @ mysql-test/r/ctype_ucs.result
  @ mysql-test/t/ctype_ucs.test
  Adding tests

  @ sql/item_func.cc
  "cs" initialization was wrong (res does not necessarily point to &str_value)

  @ sql/item_strfunc.cc
  Item_func_dec_encrypt::val_str() and Item_func_des_descrypt::val_str()
  did not set character set for tmp_value (the returned value),
  so the old value, which was previously copied from args[1]->val_str(),
  was incorrectly returned with tmp_value.