possible fix for hapi-auth-hawk payloadHash crash

In my testing it works as expected, but this issue is hard to reproduce.

The actual change is:

https://github.com/dannycoates/hapi-auth-hawk/commit/f9d44ce20362b19c089b36e96af7acdbdeae2f65

I've also reported the issue upstream https://github.com/spumko/hapi-auth-hawk/issues/2

fixes #700