Arg_comparator uses Item_cache objects to store constants being compared when
they're need a type conversion. Because this cache wasn't initialized properly
Arg_comparator might produce wrong comparison result.

The Arg_comparator::cache_converted_constant function now initializes cache
prior to usage.

mysql-test/r/select.result:
  Added a test case for he bug#49489.
mysql-test/t/select.test:
  Added a test case for he bug#49489.
sql/item_cmpfunc.cc:
  Bug#49489: Uninitialized cache led to a wrong result.
  The Arg_comparator::cache_converted_constant function now initializes cache
  prior to usage.