Subtract with overflow when first line is empty

[nthuemmel-scontain]

Given the following simple test program:

use std::fs::File;

fn main() {
    let mut reader = easy_reader::EasyReader::new(File::open("test.txt").unwrap()).unwrap();
    reader.eof();

    while let Some(line) = reader.prev_line().unwrap() {
        println!("{}", line);
    }
}

with the following test file test.txt:

Blank line above!

(Notice that the first line is blank!)

The program panics in debug mode with a substraction overflow error:

thread 'main' panicked at 'attempt to subtract with overflow', ~/.cargo/registry/src/github.com-1ecc6299db9ec823/easy_reader-0.5.1/src/lib.rs:392:57
stack backtrace:
   0: rust_begin_unwind
             at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/panicking.rs:517:5
   1: core::panicking::panic_fmt
             at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/core/src/panicking.rs:100:14
   2: core::panicking::panic
             at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/core/src/panicking.rs:50:5
   3: easy_reader::EasyReader<R>::find_end_line
             at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/easy_reader-0.5.1/src/lib.rs:392:57
   4: easy_reader::EasyReader<R>::read_line
             at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/easy_reader-0.5.1/src/lib.rs:258:44
   5: easy_reader::EasyReader<R>::prev_line
             at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/easy_reader-0.5.1/src/lib.rs:179:9
   6: easy_reader_empty_start_line::main
             at ./src/main.rs:7:28
   7: core::ops::function::FnOnce::call_once
             at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/core/src/ops/function.rs:227:5

(In release mode, it will panic with thread 'main' panicked at 'called Result::unwrap()on anErr value: Os { code: 22, kind: InvalidInput, message: "Invalid argument" }' since overflow checks are disabled)

Tested with easy_reader 0.5.1