make rustc a bit happier

rust/dpll/cnf.rs

fn parse_dimac(dimac: &str) {
    let mut lines = dimac.lines();
    let mut num_vars = 0;
    let mut num_clauses = 0;
    let mut num_vars;
    let mut num_clauses;
    
    match lines.next() {
        None => { println!("Error: expected cnf description"); return }
fn main() {
    let input: &mut String = &mut String::new();
    io::stdin().read_to_string(input);
    parse_dimac(input)
    match io::stdin().read_to_string(input) {
        Ok(_) => { parse_dimac(input) }
        Err(e) => { println!("Error: {}", e) }
    }
}