SPARC T4 assembly pack: treat zero input length in CBC.

The problem is that OpenSSH calls EVP_Cipher, which is not as
protective as EVP_CipherUpdate. Formally speaking we ought to
do more checks in *_cipher methods, including rejecting
lengths not divisible by block size (unless ciphertext stealing
is in place). But for now I implement check for zero length in
low-level based on precedent.

PR: 3087, 2775
(cherry picked from commit 5e44c144)