mingw: avoid weird build error and -Werror

The error message is:
"'*((void*)& aBoundColumn +2)' may be used uninitialized in this function"

using gcc 4.7.0 in gnu++11 mode and boost 1.48.0 .

forms/source/component/ListBox.cxx

@@ -678,7 +678,8 @@ namespace frm
             return;
         }
 
-        ::boost::optional< sal_Int16 > aBoundColumn;
+        ::boost::optional< sal_Int16 > aBoundColumn(0);
+        aBoundColumn.reset();
         if ( m_aBoundColumn.getValueType().getTypeClass() == TypeClass_SHORT )
         {
             sal_Int16 nBoundColumn( 0 );