Fix "called Option::unwrap() on a None value" in ParserIterator

[forzafedor]

Hello

In this example I had the following problem!

use std::iter::Iterator;

use nom::bytes::complete::tag;
use nom::character::complete::alphanumeric1;
use nom::combinator::iterator;
use nom::sequence::{separated_pair, terminated};
use nom::IResult;


fn main() {

  let data = "key1:value1,key2:value2,";
  let mut nom_it = iterator(
    data,
    terminated(
      separated_pair(alphanumeric1, tag(":"), alphanumeric1),
      tag(","),
    ),
  );

  assert_eq!(Some(("key1", "value1")), nom_it.next());
  assert_eq!(Some(("key2", "value2")), nom_it.next());
  assert_eq!(None, nom_it.next());
  assert_eq!(None, nom_it.next());
  let parser_result: IResult<_, _> = nom_it.finish();
  assert_eq!(Ok(("", ())), parser_result);
}

`thread 'main' panicked at /home/user/nom/src/combinator/mod.rs:912:29:
called `Option::unwrap()` on a `None` value`

This happened because of extra calls to the next() function and
Here the state variable is set to None but is not set in these conditions

I think this behavior is incorrect.
In iterator std::iter.next() additional calls to the next function will return None. I think it would be logical for this iterator to have similar behavior.

I fixed it and added tests for this example!

Please review MR you have time, thank you!

[asibahi]

Worth mentioning I removed the panic source in my (bigger scope) iterator PR: #1841