Test calls to input_byte after close_in

Add a test demonstrating issue ocaml#11878

testsuite/tests/lib-channels/close_in.ml

@@ -0,0 +1,22 @@
+(* TEST *)
+
+(* The number of bytes we’ll rewind after closing; a value
+   between 1 and IO_BUFFER_SIZE *)
+let nb_bytes = 3
+
+let () =
+  let ic = open_in_bin Sys.argv.(0) in
+  seek_in ic nb_bytes;
+  close_in ic;
+  seek_in ic 0;
+  for _ = 1 to nb_bytes do
+    (* the bytes we get here were never initialised *)
+    ignore (input_byte ic)
+  done;
+  assert (
+    try
+      ignore (input_byte ic);
+      false
+    with
+    | Sys_error _ -> true
+    | _           -> false)