xmloff: fix crash on exporting non-loadable external image URL

As seen on exporting rhbz739243-1.odt, the xGraphic is null in that case.

(regression from 4469b29f)

Change-Id: I535d67eeddc1b9bdf2562fc3b54a97e38ab0354c

xmloff/source/text/txtparae.cxx

@@ -3024,9 +3024,12 @@ static OUString getMimeType(const OUString& sImageUrl)
     Reference< XGraphic > xGraphic( xProvider->queryGraphic( aMediaProperties ) );
 
     OUString aSourceMimeType;
-    Reference< XPropertySet > xGraphicPropertySet( xGraphic, UNO_QUERY_THROW );
-    if ( xGraphicPropertySet->getPropertyValue( "MimeType" ) >>= aSourceMimeType )
+    Reference<XPropertySet> const xGraphicPropertySet(xGraphic, UNO_QUERY);
+    if (xGraphicPropertySet.is() && // it's null if it's an external link
+        (xGraphicPropertySet->getPropertyValue("MimeType") >>= aSourceMimeType))
+    {
         return aSourceMimeType;
+    }
     return OUString("");
 }