Shrink boot2 by a couple more bytes.

Reviewed by:    jhb
Tested by:      me, dim
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