BUG/MEDIUM: connection: don't forget to always delete the list's head

BUG/MEDIUM: connection: don't forget to always delete the list's head

During a test it happened that a connection was deleted before the
stream it's attached to, resulting in a crash related to the fix
18a85fe ("BUG/MEDIUM: streams: Don't forget to remove the si from
the wait list.") during the LIST_DEL(). Make sure to always delete
the list's head in this case so that other elements can safely
detach later.

This is purely 1.9, no backport is needed.