fdo#54648: WW8 export: test that FlyFrm actually has layout frm

The bugdoc has 2 "hidden" drawing objects that are invisible.

Change-Id: I58179f2e620348db5357b38f834e1edca1f20ae2

sw/source/filter/ww8/wrtw8esh.cxx

@@ -1648,6 +1648,10 @@ bool lcl_isInHeader(const SwFrmFmt& rFmt)
     if (!pFlyFrmFmt)
         return false;
     SwFlyFrm* pFlyFrm = const_cast<SwFlyFrm*>(pFlyFrmFmt->GetFrm());
+    if (!pFlyFrm) // fdo#54648: "hidden" drawing object has no layout frame
+    {
+        return false;
+    }
     SwPageFrm* pPageFrm = pFlyFrm->FindPageFrmOfAnchor();
     SwFrm* pHeader = pPageFrm->Lower();
     if (pHeader->GetType() == FRM_HEADER)