and related small fixes.

mysql-test/t/user_var.test:
  test for bug
sql/field_conv.cc:
  From the C standard, memcpy() has undefined behaviour if to->ptr==from->ptr
sql/item_func.cc:
  In the case of BUG#56138, entry->value==ptr in which case memcpy()
  has undefined results per the C standard.
sql/sql_select.cc:
  Work around a bug in old gcc