Fix (and test) panic when our counterparty uses a bogus funding tx

During the block API refactor, we started calling
Channel::force_shutdown when a channel is closed due to a bogus
funding tx. However, we still set the channel's state to Shutdown
prior to doing so, leading to an assertion in force_shutdown (that
the channel is not already closed).

This removes the state-set call and adds a (long-overdue) test for
this case.

Fixes: 60b962a18ebcf494340ddc001870f8160c625968