Document that EVP_EncodeBlock writes a trailing NUL.

Fix base64_test.c to account for this.

Change-Id: I0b3e8062a2130fb01a7e6f175968484769c406f9
Reviewed-on: https://boringssl-review.googlesource.com/1250
Reviewed-by: Adam Langley <agl@google.com>
2 files changed