Fixed (NULL + 0) undefined behaviour

Fixed (NULL + 0) undefined behaviour

It appears that arithmetic on NULL pointers is undefined, even when we
just add zero.

Monocypher generally allows input buffers to be NULL pointers if their
length is zero.  This is because we never dereference those pointers in
this case.  Likewise, we should not perform any arithmetic on them.

The fix is to return immediately when the input buffer length is zero.