When I switched the base64 code to use size_t, I missed that one of the
loops was counting down, not up, and depended on the loop variable going
negative.

Additionally this change fixes a bug in NETSCAPE_SPKI_b64_encode where
the size of the result buffer was incorrectly calculated and a possible
memory leak.

Change-Id: Ibdf644244291274f50b314f3bb13a61b46858ca1
Reviewed-on: https://boringssl-review.googlesource.com/1220
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>