There's an assertion failure that can occur if a connection has
optimistic data waiting, and then the connect() call returns 0 on the
first attempt (rather than -1 and EINPROGRESS).  That latter behavior
from connect() appears to be an (Open?)BSDism when dealing with remote
addresses in some cases. (At least, I've only seen it reported with
the BSDs under libevent, even when the address was 127.0.0.1.  And
we've only seen this problem in Tor with OpenBSD.)

Fixes bug 9017; bugfix on 0.2.3.1-alpha, which first introduced
optimistic data. (Although you could also argue that the commented-out
connection_start_writing in 155c9b80 back in 2002 is the real source
of the issue.)