try not to call memcpy with length 0 in hash calculations

try not to call memcpy with length 0 in hash calculations

memcpy is marked as nonnull for its input, but ignores the input anyhow
if the declared length is zero. Our SHA2 implementations do this as
well, it was "just" MD5 and SHA1 missing, so we add the length check
here as well as along the callstack as it is really pointless to do all
these method calls for "nothing".

Reported-By: gcc -fsanitize=undefined