Nullify own and threshold pointers before use

Even though they are never used in an undefined way per se, there is an
associated(node, threshold_node) statement which compares a valid
pointer to the undefined state of threshold_node (or own_node), which
could potentially erroneously evaluate to true, should threshold_node
accidentally hold precisely the address of a valid node.

ftimings/ftimings.F90

@@ -1082,6 +1082,8 @@ module ftimings
     logical :: is_sorted_act, print_own, print_threshold
     integer :: unit_act
 
+    nullify(own_node)
+    nullify(threshold_node)
 
     if (present(threshold)) then
       threshold_act = threshold