sw: InsertCnt_(): don't call FindPrevCnt() when it doesn't make sense

When pasting a table we get:

warn:legacy.osl:29407:29407:sw/source/core/layout/findfrm.cxx:1136: <SwFrame::FindPrevCnt_()> - unknown layout situation: current frame should be in page header or page foote

... because SwCellFrame's constructor inserts the content of the cell
before the SwCellFrame's mpUpper is set, so it's not connected to
anything; hence FindNextCnt()/FindPrevCnt() can only return frames
inside the cell anyway, and all of those are newly created, so
invalidating a11y flow-relations for them seems pointless because a11y
doesn't even know about them yet.

Change-Id: I96eb59d6ecfdaa453894d06494c6c50faa7cd1bd
Reviewed-on: https://gerrit.libreoffice.org/73046
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sw/source/core/layout/frmtool.cxx

@@ -1431,7 +1431,8 @@ void InsertCnt_( SwLayoutFrame *pLay, SwDoc *pDoc,
                 // no notification, if <SwViewShell> is in construction
                 if ( pViewShell && !pViewShell->IsInConstructor() &&
                      pViewShell->GetLayout() &&
-                     pViewShell->GetLayout()->IsAnyShellAccessible() )
+                     pViewShell->GetLayout()->IsAnyShellAccessible() &&
+                     pFrame->FindPageFrame() != nullptr)
                 {
                     pViewShell->InvalidateAccessibleParaFlowRelation(
                         dynamic_cast<SwTextFrame*>(pFrame->FindNextCnt( true )),
@@ -1512,7 +1513,8 @@ void InsertCnt_( SwLayoutFrame *pLay, SwDoc *pDoc,
                 // no notification, if <SwViewShell> is in construction
                 if ( pViewShell && !pViewShell->IsInConstructor() &&
                      pViewShell->GetLayout() &&
-                     pViewShell->GetLayout()->IsAnyShellAccessible() )
+                     pViewShell->GetLayout()->IsAnyShellAccessible() &&
+                     pFrame->FindPageFrame() != nullptr)
                 {
                     pViewShell->InvalidateAccessibleParaFlowRelation(
                             dynamic_cast<SwTextFrame*>(pFrame->FindNextCnt( true )),
@@ -1592,7 +1594,8 @@ void InsertCnt_( SwLayoutFrame *pLay, SwDoc *pDoc,
                     // no notification, if <SwViewShell> is in construction
                     if ( pViewShell && !pViewShell->IsInConstructor() &&
                          pViewShell->GetLayout() &&
-                         pViewShell->GetLayout()->IsAnyShellAccessible() )
+                         pViewShell->GetLayout()->IsAnyShellAccessible() &&
+                         pFrame->FindPageFrame() != nullptr)
                     {
                         pViewShell->InvalidateAccessibleParaFlowRelation(
                             dynamic_cast<SwTextFrame*>(pFrame->FindNextCnt( true )),