Add fix for NPE on encrypt/decrypt

[coltfred]

Fixes #167

There are a couple cases where the cipher.update can return null. One is if the payload is too short for a block and another is to support GCM tag verification in the context of FIPS.

The comment says:

the new buffer with the result, or null if this cipher is a block cipher and the input data is too short to result in a new block.

So either way we should guard against it. I wrote a custom provider to use in the test that buffers everything internally just to show that it works if update returns null. Messing with FIPS for testing was something I didn't want to take on.