sd-netlink: do not trigger assertion by calling socket_broadcast_group_unref() with...

sd-netlink: do not trigger assertion by calling socket_broadcast_group_unref() with an arbitrary group number

socket_broadcast_group_unref() is only called in netlink_slot_disconnect(),
so the assertion should not be triggered as the match slot was
successfully created.

But, we usually design `_ref/unref()` functions as they can be called
for any inputs. So, let's also follow the design rule here.