core/device: Fix crash when enabling experimental

This fixes the following trace:

Invalid read of size 8
   at 0x53BE1B1: ??? (in /usr/lib64/libdbus-1.so.3.15.2)
   by 0x53AAB45: _dbus_type_writer_write_fixed_multi (in /usr/lib64/libdbus-1.so.3.15.2)
   by 0x49E629: dev_property_get_flags (device.c:953)
   by 0x4C611E: append_property.isra.0 (object.c:511)
   by 0x4C6C3D: append_properties (object.c:540)
   by 0x4C6CD2: append_interface (object.c:555)
   by 0x50E740C: g_slist_foreach (in /usr/lib64/libglib-2.0.so.0.4800.2)
   by 0x4C734E: append_interfaces (object.c:1111)
   by 0x4C734E: append_object (object.c:1126)
   by 0x50E740C: g_slist_foreach (in /usr/lib64/libglib-2.0.so.0.4800.2)
   by 0x4C7398: append_object (object.c:1129)
   by 0x50E740C: g_slist_foreach (in /usr/lib64/libglib-2.0.so.0.4800.2)
   by 0x4C7398: append_object (object.c:1129)
1 file changed