[PKT_SCHED]: Correctly handle empty ematch trees

Fixes an invalid memory reference when the basic classifier
is used without any ematches but just actions.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/ematch.c

@@ -298,6 +298,11 @@ int tcf_em_tree_validate(struct tcf_proto *tp, struct rtattr *rta,
 	struct tcf_ematch_tree_hdr *tree_hdr;
 	struct tcf_ematch *em;
 
+	if (!rta) {
+		memset(tree, 0, sizeof(*tree));
+		return 0;
+	}
+
 	if (rtattr_parse_nested(tb, TCA_EMATCH_TREE_MAX, rta) < 0)
 		goto errout;