cmp_item_sort_string::cmp() wasn't checking values_res variable for null.
Later called function was dereferenced it and crashed server.

Added null check to cmp_item_sort_string::cmp().



sql/item_cmpfunc.h:
  Fix bug#15268  Unchecked null value caused server crash
  Added null check to cmp_item_sort_string::cmp().
mysql-test/t/select.test:
  Test case for bug#15268 Unchecked null value caused server crash
mysql-test/r/select.result:
  Test case for bug#15268 Unchecked null value caused server crash