libedit: Avoid out of bounds read in 'bind' command

This is CVS revision 1.31 from NetBSD lib/libedit/chartype.c:
Make sure that argv is NULL terminated since functions like tty_stty rely
on it to be so (Gerry Swinslow)

This broke when the wide-character support was enabled in libedit. The
conversion from multibyte to wide-character did not supply the apparently
expected terminating NULL in the new argv array.

PR: 233343
Submitted by: Yuichiro NAITO
Obtained from: NetBSD
MFC after: 1 week