type conversion.

Instead of copying of whole character string from a temporary
buffer, the server copied a short-living pointer to that string
into a long-living structure. That has been fixed.


mysql-test/r/select.result:
  Added test case for bug#33764.
mysql-test/t/select.test:
  Added test case for bug#33764.
sql/item_cmpfunc.cc:
  Fixed bug#33764.
  Copying of a pointer has been replaced with an optional copying of
  a whole array to a newly allocated memory space in case of a
  functional source item.