fixed rubout of Q-Register specifications

* This was a regression introduced by 257a0bf128e109442dce91c4aaa1d97fed17ad1a.
* The undo token that frees newly allocated teco_machine_qregspec_t must actually
  reset the pointer as well since any subsequent token, pushed by teco_undo_qregspec_own(),
  will expect a valid pointer.
* Could have been done via
  ctx->expectqreg = NULL;
  teco_undo_qregspec_own(ctx->expectqreg);
  but using a special clear function requires less memory and is easier to understand.
* Added test case. This wouldn't always crash, but should definitely show up in Valgrind.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/undo.h b/src/undo.h
index 6fbb1c1..459fdb0 100644
--- a/src/undo.h
+++ b/src/undo.h
@@ -143,6 +143,7 @@ gpointer teco_undo_push_size(teco_undo_action_t action_cb, gsize size)
  * At undo-time however, it is expected to be valid.
  * Therefore it is crucial to set the variable afterwards or at least to nullify it.
  * The DELETE function may consequently have to deal with NULL pointers.
+ * Also, any other undo action must always leave the variable in a valid state.
  *
  * @ingroup undo_objects
  */