Test and fix removal at past-the-end index

tests/arrayvec.rs

@@ -123,6 +123,14 @@ fn ArrayVec_remove() {
   assert_eq!(&av[..], &[1, 3][..]);
 }
 
+#[test]
+#[should_panic]
+fn ArrayVec_remove_invalid() {
+  let mut av: ArrayVec<[i32; 1]> = Default::default();
+  av.push(1);
+  av.remove(1);
+}
+
 #[test]
 fn ArrayVec_swap_remove() {
   let mut av: ArrayVec<[i32; 10]> = Default::default();