Problem: INTERVAL function implementation doesn't handle NULL range values.

Fix: skip NULL ranges looking for a proper one.


mysql-test/r/func_set.result:
  Fix for bug #32560: crash with interval function and count(*)
    - test result.
mysql-test/t/func_set.test:
  Fix for bug #32560: crash with interval function and count(*)
    - test case.
sql/item_cmpfunc.cc:
  Fix for bug #32560: crash with interval function and count(*)
    - skip NULL ranges calculating INTERVAL(...).