handling of padding. This fixes a crash that can be triggered by feeding base64 data followed by 64 or more padding characters, which results in a negative output length. This issue was reported by David Ramos, although the same bug has been sitting in the OpenSSL RT since 2011: https://rt.openssl.org/Ticket/Display.html?id=2608 Worse still, BIO_read seems to be completely unable to detect that the base64 input was invalid/corrupt - in particular, enabling BIO_FLAGS_BASE64_NO_NL results in a stream of zero value bytes rather than no input (possibly a good replacement for /dev/null...), which could result in nasty consequences. Prior to this fix some zero value bytes were also injected without this flag being enabled. The recently added base64 regress triggers and documents these issues (and also ensures that this change retains functional behaviour).