Fix assert in sc/undoblk3

since:
commit 0abf2ac27859f1e91a3d183b81948b1fe8f5aca3
Author: Caolán McNamara <caolanm@redhat.com>
Date:   Wed Sep 16 11:25:25 2020 +0100

    AppendContentsIfInRefDoc never passed a null ScDocument*

    Change-Id: Ia21f8fecdf8a6aea7ea9c6ea4d579c5e53882bbd
    Reviewed-on: https://gerrit.libreoffice.org/c/core/+/102982
    Tested-by: Caolán McNamara <caolanm@redhat.com>
    Reviewed-by: Caolán McNamara <caolanm@redhat.com>

Change-Id: Id699c7be3a81b251541f9a46fd3e1149c47f5df9
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/121642
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
diff --git a/sc/source/ui/undo/undoblk3.cxx b/sc/source/ui/undo/undoblk3.cxx
index 40f5741..073035b 100644
--- a/sc/source/ui/undo/undoblk3.cxx
+++ b/sc/source/ui/undo/undoblk3.cxx
@@ -1286,7 +1286,7 @@ pRedoDoc    ( std::move(pNewRedoDoc) ),
aRange      ( aMarkRange ),
bMulti      ( bNewMulti )
{
    assert(pUndoDoc && pUndoDoc);
    assert(pUndoDoc && pRedoDoc);
    SetChangeTrack();
}