Not telling the cmux would sometimes cause an assertion failure in
relay.c when we tried to get an active circuit and found an "active"
circuit with no cells.

Additionally, replace that assert with a test and a log message.

Fix for bug 20203. This is actually probably a bugfix on
0.2.8.1-alpha, specifically my code in 8b4e5b7e where I
made circuit_mark_for_close_() do less in order to simplify our call
graph. Thanks to "cypherpunks" for help diagnosing.