detect_preamble and in extension from_reader_with_all_options doesn't check for size

[DerOrfa]

In detect_preamble in mem.rs

        if &buf[0..4] == b"DICM" {
            return Ok(ReadPreamble::Never);
        }

is done without checking if buf actually has at least 4 bytes, leading to panic when, for example, trying to read empty files with from_reader_with_all_options and friends.

Should return with io::Error.

[Enet4]

Thank you for reporting! Would you be able to send in a pull request?