Unexpected characters received (maybe typed by the user) were supposed to
be stuffed in the keyboard input buffer using ungetstr(), but the input string
was cleared rather than NUL-terminated before logging the expected character
and calling ungetstr(), so nothing (a blank string) was logged and nothing was
stuffed in the keyboard input-buffer in this case. Just an off-by-one bug.

This should fix issue #304. Thanks to Keyop and Nelgin for their persistence
in reporting and testing (in #synchronet at irc.synchro.net).