Fix EVP_DecodeBlock and add tests.

Another signedness error. Leave a TODO to possibly resolve EVP_DecodeBlock's
ignoring padding. Document some of the Init/Update/Finish versions' behavior.

Change-Id: I78a72c3163f8543172a7008b2d09fb10e003d957
Reviewed-on: https://boringssl-review.googlesource.com/1230
Reviewed-by: Adam Langley <agl@google.com>