fdo#68956: fix string conversion

Added the SetText method to make it absolutely clear.

Regression from 150c9f8b

Change-Id: I4e9935fd13fca8f9cfdfda65c980d08b1e85b459
Reviewed-on: https://gerrit.libreoffice.org/5819Reviewed-by: Marcos Souza <marcos.souza.org@gmail.com>
Reviewed-by: Thomas Arnhold <thomas@arnhold.org>
Tested-by: Thomas Arnhold <thomas@arnhold.org>

starmath/source/mathmlimport.cxx

@@ -1192,7 +1192,7 @@ void SmXMLAnnotationContext_Impl::StartElement(const uno::Reference<
 void SmXMLAnnotationContext_Impl::Characters(const OUString &rChars)
 {
     if (bIsStarMath)
-        GetSmImport().GetText() + rChars;
+        GetSmImport().SetText( GetSmImport().GetText() + rChars );
 }
 
 ////////////////////////////////////////////////////////////

starmath/source/mathmlimport.hxx

@@ -255,8 +255,9 @@ public:
         return result;
     }
 
-    sal_Bool GetSuccess()           { return bSuccess; }
-    OUString &GetText()             { return aText; }
+    sal_Bool GetSuccess()              { return bSuccess; }
+    OUString GetText()                 { return aText; }
+    void SetText(const OUString &rStr) { aText = rStr; }
 
     virtual void SetViewSettings(const com::sun::star::uno::Sequence<com::sun::star::beans::PropertyValue>& aViewProps);
     virtual void SetConfigurationSettings(const com::sun::star::uno::Sequence<com::sun::star::beans::PropertyValue>& aViewProps);