Function check_range 

pub fn check_range(
    path: &str,
    buf_len: usize,
    range: Range<usize>,
    what: &str,
) -> Result<()>

Bounds check for a field read out of a buffer whose length came from the file. Format code calls this before slicing so that a bad count is a Error::Corrupt a caller can catch, not a panic it cannot.