remodel SubrangeAdaptor<char*>
remodel SubrangeAdaptor<char*>

if we are at the end of the input and there is less than one block
remaining make a copy of the block into a zero filled array instead of
dynamically returning zeros. This is less fragile but we have an
addiotinal call to std::vector::empty() for every invocation of operator[].

mirco/libblake2,mirco/libblake2