dbus_message_iter_get_signature: Fix two memory leaks on OOM

dbus_message_iter_get_signature: Fix two memory leaks on OOM

Previously,  `retstr` would not be freed when `_dbus_string_append_len()`
or `_dbus_string_steal_data()` failed.

Fix those by:
 * jumping to `_dbus_string_free()` when `_dbus_string_append_len()` fails
 * ignoring the return value of `_dbus_string_free()`.

The latter works because in case of failure, `ret` will be set
to NULL by `_dbus_string_steal_data()`.

Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>