fix base64.c "overrun"

[Ted Unangst]

>From inspection, it appears FreeBSD's libc base64.c will sometimes
erroneously fail to decode a base64 string into a precisely sized
buffer. The overflow check is a little too greedy. The same bug was
fixed in OpenBSD; it would be helpful if FreeBSD adopted the fix as
well. :)

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