[PKT_SCHED] ematch: tcf_em_destroy robustness

Make the code in tcf_em_tree_destroy more robust and cleaner:
 * Don't need to cast pointer to kfree() or avoid passing NULL.
 * After freeing the tree, clear the pointer to avoid possible problems
from repeated free.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>