(RFC) Crashfix: Do not feed certain bogus utf8 into python bindings

[michaelortmann]

Found by: BigBadWouf Patch by: michaelortmann Fixes: #1711

One-line summary: Should fix certain crashes like the one reported via #1711

Additional description (if needed):

Test cases demonstrating functionality (if applicable): Instead of crashing, log like:

[08:22:27] [@] :testuser!~michael@localhost PRIVMSG #foo :
                                                          [08:22:27] bogus utf8, python bind "*python:pubm:74062bafa29" not triggered