fix: fmt crashing with comment at end of file

closes #568

crates/aiken-lang/src/parser/extra.rs

@@ -24,7 +24,7 @@ pub struct Comment<'a> {
 impl<'a> From<(&Span, &'a str)> for Comment<'a> {
     fn from(src: (&Span, &'a str)) -> Comment<'a> {
         fn char_indice(s: &str, i: usize) -> usize {
-            s.char_indices().nth(i).expect("char at given indice").0
+            s.char_indices().nth(i).unwrap_or((i, ' ')).0
         }
 
         let start = char_indice(src.1, src.0.start);

crates/aiken-lang/src/tests/format.rs

@@ -17,6 +17,23 @@ fn assert_fmt(src: &str, expected: &str) {
     assert_eq!(out, out2, "formatting isn't idempotent");
 }
 
+#[test]
+fn comment_at_end_of_file() {
+    let input = indoc! { r#"
+      type Foo =
+        Int
+
+      //"#};
+
+    let output = indoc! { r#"
+      type Foo =
+        Int
+      //
+    "#};
+
+    assert_fmt(input, output);
+}
+
 #[test]
 fn test_format_if() {
     let src = indoc! {r#"