> I've never used negative terminators in asynchat and I'm not even sure why one would want to. 
no one wants :), but terminator numeric value may be achieved from the net, and hackers sometimes use such technique.

attached shorttest.py do the test. If negative terminator is passed, ValueError exception will raise on patched version, and will not raise in unpatched.

Currently, I studying test system for python, but I think you will add such simple test faster and cleaner.