mkhash currently returns the hash of an empty input when trying to hash
a folder. This can be missleading in caseswhere e.g. an env variable is
undefined which should contain a filename. `mkhash ./path/to/$FILE`
would exit with code 0 and return a legit looking checksum.
A better behaviour would be to fail with exit code 1, which imitates the
behaviour of `md5sum` and `sha256sum`.
To avoid hashing of folders the `stat()` is checked.
Hashing empty inputs result in the following checksums:
md5: d41d8cd98f00b204e9800998ecf8427e
sha256: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
Signed-off-by: Paul Spooren <mail@aparcar.org>
