Fix NULL pointer reference when using basic match

If basic match has no tree of matches underneath
then print_ematch would core dump.

tc/m_ematch.c

@@ -449,12 +449,14 @@ static int print_ematch_list(FILE *fd, struct tcf_ematch_tree_hdr *hdr,
 	if (tb == NULL)
 		return -1;
 
-	if (parse_rtattr_nested(tb, hdr->nmatches, rta) < 0)
-		goto errout;
+	if (hdr->nmatches > 0) {
+		if (parse_rtattr_nested(tb, hdr->nmatches, rta) < 0)
+			goto errout;
 
-	fprintf(fd, "\n  ");
-	if (print_ematch_seq(fd, tb, 1, 1) < 0)
-		goto errout;
+		fprintf(fd, "\n  ");
+		if (print_ematch_seq(fd, tb, 1, 1) < 0)
+			goto errout;
+	}
 
 	err = 0;
 errout: