fix comment and add test for many_index oob

Author: NiklasJonsson

src/map.rs

@@ -832,7 +832,6 @@ impl<K, V, S> IndexMap<K, V, S> {
         indices: [usize; N],
     ) -> Option<[(&K, &mut V); N]> {
         // SAFETY: Can't allow duplicate indices as we would return several mutable refs to the same data.
-        // Additionally, handle out-of-bounds indices (internal error in get_index_of) with panic.
         let len = self.len();
         for i in 0..N {
             let idx = indices[i];

src/map/tests.rs

@@ -479,3 +479,11 @@ fn many_mut_multi_fail_duplicate() {
         None
     );
 }
+
+#[test]
+fn many_index_mut_fail_oob() {
+    let mut map: IndexMap<u32, u32> = IndexMap::default();
+    map.insert(1, 10);
+    map.insert(321, 20);
+    assert_eq!(map.get_many_index_mut([1, 3]), None);
+}