fix for base64.c "overrun"

[Ted Unangst <tedu%tedunangst.com@localhost>]

>From inspection, it appears that base64.c in libc will erroneously
detect a pending overflow when pton decodes a base64 string into a
precisely sized buffer. The same bug existed in OpenBSD; it would be
nice if it were fixed in NetBSD as well.

http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/lib/libc/net/base64.c.diff?r1=1.6&r2=1.7