libceph: Fix base64-decoding when input ends in newline.

It used to return -EINVAL because it thought the end was not aligned
to 4 bytes.

Clean up superfluous src < end test in if, the while itself guarantees
that.

Signed-off-by: Tommi Virtanen <tommi.virtanen@dreamhost.com>
Signed-off-by: Sage Weil <sage@newdream.net>