sw: fix crash on pasting from an already closed document

As pointed out by
<http://crashreport.libreoffice.org/stats/signature/%60anonymous%20namespace'::lcl_checkClassification%28SwDoc%20*,SwDoc%20*%29+0x0>.
Thanks to Michael Stahl for figuring out the list of steps to reproduce
this bug:

1) Create a new Writer document
2) Type some text
3) Select all
4) Copy it
5) Close the document
6) Create a second Writer document
7) Paste -> crash

Change-Id: I8cfcad465803e37ec9d01d035428c5ca953adf14
Reviewed-on: https://gerrit.libreoffice.org/26380
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>
diff --git a/sw/source/uibase/dochdl/swdtflvr.cxx b/sw/source/uibase/dochdl/swdtflvr.cxx
index c6b6da2..44db22d 100644
--- a/sw/source/uibase/dochdl/swdtflvr.cxx
+++ b/sw/source/uibase/dochdl/swdtflvr.cxx
@@ -3286,7 +3286,8 @@ bool SwTransferable::PrivatePaste( SwWrtShell& rShell )
    }

    bool bRet = true;
    if (lcl_checkClassification(m_pWrtShell->GetDoc(), rShell.GetDoc()))
    // m_pWrtShell is nullptr when the source document is closed already.
    if (!m_pWrtShell || lcl_checkClassification(m_pWrtShell->GetDoc(), rShell.GetDoc()))
        bRet = rShell.Paste(m_pClpDocFac->GetDoc());

    if( bKillPaMs )