Blake2b, Poly1305: fixed undefined behaviour

Blake2b, Poly1305: fixed undefined behaviour

Caugth by TIS-CI and the latest Clang's UBSan.
Incrementing a NULL pointer, even by a NULL offset, is not permitted.
This was caused by the removal of a conditional that exited early if the
message was empty.
The fix was to move the increment inside the alignment loop.

It may be tiny bit slower, but this was the slow path already.
Users can avoid it by aligning their increments to block boundaries.