BUG/MINOR: jwt: Double free in deinit function

The node pointer was not moving properly along the jwt_cert_tree during
the deinit which ended in a double free during cleanup (or when checking
a configuration that used the jwt_verify converter with an explicit
certificate specified).

This patch fixes GitHub issue #1533.
It should be backported to 2.5.

src/jwt.c

@@ -363,6 +363,7 @@ static void jwt_deinit(void)
 	node = ebmb_first(&jwt_cert_tree);
 	while (node) {
 		entry = ebmb_entry(node, struct jwt_cert_tree_entry, node);
+		ebmb_delete(node);
 		ha_free(&entry);
 		node = ebmb_first(&jwt_cert_tree);
 	}