fdo#48778: PPTX import crashes.

This probably occurs when the document has wrong idx (?)
(cherry picked from commit 11713989b1580799635fff997b920d9bd4bf619f)

Signed-off-by: Michael Stahl <mstahl@redhat.com>
diff --git a/oox/source/ppt/slidefragmenthandler.cxx b/oox/source/ppt/slidefragmenthandler.cxx
index 4652690..59844ef 100644
--- a/oox/source/ppt/slidefragmenthandler.cxx
+++ b/oox/source/ppt/slidefragmenthandler.cxx
@@ -140,8 +140,8 @@ SlideFragmentHandler::~SlideFragmentHandler() throw()

    case PPT_TOKEN( bgRef ):            // a:CT_StyleMatrixReference
        {
            FillPropertiesPtr pFillPropertiesPtr( new FillProperties(
                *mpSlidePersistPtr->getTheme()->getFillStyle( rAttribs.getInteger( XML_idx, -1 ) ) ) );
            const FillProperties *pFillProperties = mpSlidePersistPtr->getTheme()->getFillStyle( rAttribs.getInteger( XML_idx, -1 ) );
            FillPropertiesPtr pFillPropertiesPtr( pFillProperties ? new FillProperties( *pFillProperties ) : new FillProperties() );
            ContextHandlerRef ret = new ColorContext( *this, mpSlidePersistPtr->getBackgroundColor() );
            mpSlidePersistPtr->setBackgroundProperties( pFillPropertiesPtr );
            return ret;