Cleanups.

Cleanups.

Attempt to allocate the buffer before doing any decryption calls and, if
the attempt fails, report an error and return an error indication.

Make sizes unsigned, as they can't be negative;
EVP_CIPHER_CTX_block_size() returns an int, but that's just because they
picked the wrong data type in OpenSSL.

No need to use calloc() for the output buffer - just use malloc().