Non-empty stack overflow table

[plafer]

The following test fails with error OutputStackOverflow(1), presumably because there were 2 PUSH instructions, and one ADD (which shifts stack left), and so the overflow table contains a 0 entry at the end and is contained non-empty. This is probably a new behavior since #1456.

I'm not sure why this doesn't occur in more tests yet, but it seems like the behavior we want is: if a 0 is pushed onto the stack overflow table, and the table is empty, then don't populate the overflow table (since when we shift right on an empty overflow table, we populate s[15] with 0).

#[test]
fn test() {
    use crate::{prove, Assembler, DefaultHost, Program, ProvingOptions, StackInputs};
    // instantiate the assembler
    let mut assembler = Assembler::default();

    // this is our program, we compile it from assembly code
    let program = assembler.assemble_program("begin push.3 push.5 add end").unwrap();

    // let's execute it and generate a STARK proof
    let (outputs, proof) = prove(
        &program,
        StackInputs::default(),    // we won't provide any inputs
        DefaultHost::default(),    // we'll be using a default host
        ProvingOptions::default(), // we'll be using default options
    )
    .unwrap();

    // the output should be 8
    assert_eq!(8, outputs.first().unwrap().as_int());
}