These SwTxtFrm calls shall apparently be done via pOwn, not m_rThis

...given the initialization of pOwn based on m_rThis at the start of
CalcUpperSpace and the pOwn->IsTxtFrm() checks preceeding these casts.

At least CppunitTest_sw_odfimport via

SwFlowFrm::CalcUpperSpace
SwSectionFrm::Format
SwLayoutFrm::MakeAll
SwSectionFrm::MakeAll
SwFrm::PrepareMake
SwFrm::Calc
SwLayAction::FormatLayout
SwLayAction::FormatLayout
SwLayAction::FormatLayout
SwLayAction::InternalAction
SwLayAction::Action
SwViewShell::ImplEndAction
SwViewShell::EndAction
SwCrsrShell::EndAction
SwView::OuterResizePixel
SfxViewFrame::DoAdjustPosSizePixel
SfxViewFrame::Resize
SfxFrameViewWindow_Impl::Resize
Window::ImplCallResize
Window::Show
SfxBaseController::ConnectSfxFrame_Impl
SfxBaseController::attachFrame
(anonymous namespace)::SfxFrameLoader_Impl::impl
(anonymous namespace)::SfxFrameLoader_Impl::load
framework::LoadEnv::impl_loadContent
framework::LoadEnv::startLoading
framework::LoadEnv::loadComponentFromURL
unotest::MacrosTest::loadFromDesktop
SwModelTestBase::load
SwModelTestBase::executeImportTest
testFdo55814::Import

leads to calls where m_rThis is not a SwTxtFrm.

As this mixed usage of m_rThis and pOwn is in there since
84a3db80b4fd66c6854b3135b5f69b61fd828e62 "initial import" it is hard to tell
whether further uses of m_rThis should be replace with uses of pOwn, too.

Change-Id: I6a59aa3aec2d28b8aec13cd4b9528fdbe4ab093a