GROUPING BY FUNCTIONS.... (PART

The bug was introduced in a patch for bug 49897.

Problem: The assertion inserted by the original patch to guard against
zero-lenght sort keys during merge phase triggers also when the whole
set fits in memory.

Fix: Move assert so that it does not trigger if the whole set is in
memory.


mysql-test/r/group_by.result:
  Add test for bug#11765254
mysql-test/t/group_by.test:
  Add test for bug#11765254
sql/filesort.cc:
  Move assertion