Fix b64_pton output buffer overrun test for exact-sized buffer

b64_pton would sometimes erroneously fail to decode a base64 string into
a precisely sized buffer. The overflow check was a little too greedy.

Reported by: Ted Unangst on freebsd-hackers@
Reviewed by: loos, trasz
Obtained from: OpenBSD
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D1218