tools: ynl: don't leak mcast_groups on init error

tools: ynl: don't leak mcast_groups on init error

Make sure to free the already-parsed mcast_groups if
we don't get an ack from the kernel when reading family info.
This is part of the ynl_sock_create() error path, so we won't
get a call to ynl_sock_destroy() to free them later.

Fixes: 86878f14d71a ("tools: ynl: user space helpers")
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20240220161112.2735195-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>