Null terminate values returned by event_notification_get_string.
We never documented whether returned strings for values whose length is >= the buffer length passed in are null-terminated or not. Previously, there were not, but most of our callers never checked. Hence, we now force null termination. Note that if someone passes a bogus length value, this will now blow up where it formerly might not have. I consider this a value-added debugging feature... Note also that this is unrelated to recent stated problems, but turned up while looking for that bug!