chardev/char-socket: handle NULL addr in char_socket_get_addr

When called on an unrealized chardev (e.g. from
qmp_qom_list_properties), s->addr is NULL. Return an error instead of
assert().

Fixes: 123676e989 ("char-socket: add 'addr' property")
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

chardev/char-socket.c

@@ -1529,6 +1529,10 @@ char_socket_get_addr(Object *obj, Visitor *v, const char *name,
 {
     SocketChardev *s = SOCKET_CHARDEV(obj);
 
+    if (!s->addr) {
+        error_setg(errp, "socket not connected");
+        return;
+    }
     visit_type_SocketAddress(v, name, &s->addr, errp);
 }