reset border should clear all borders

If no borders were originally defined, the reset button was failing
to clear the last-selected border line.

Eg. page border, click on two of the borders, then hit reset. One border
remains defined, and didn't reset to the starting value of non-visible.
Because one border was still "selected", it became visible again as
SelStyleHdl_Impl reinitialized.

Change-Id: I2d20ae1e02816d5b30fe126572b54e7083114fc3
Reviewed-on: https://gerrit.libreoffice.org/30365
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Justin Luth <justin_luth@sil.org>

cui/source/tabpages/border.cxx

@@ -527,6 +527,8 @@ void SvxBorderTabPage::Reset( const SfxItemSet* rSet )
         m_pFrameSel->HideAllBorders();
     }
 
+    if( !m_pFrameSel->IsAnyBorderVisible() )
+        m_pFrameSel->DeselectAllBorders();
 
     // depict line (color) in controllers if unambiguous: