Revert "This probably is not worth displaying a warning about"

It might be a useful warning.

This reverts commit d202f85171.

svl/source/items/itempool.cxx

@@ -425,6 +425,7 @@ void SfxItemPool::SetSecondaryPool( SfxItemPool *pPool )
                         for( size_t i = pItemArr->size(); i; ++ppHtArr, --i )
                             if ( !(*ppHtArr) )
                             {
+                                OSL_FAIL( "old secondary pool must be empty" );
                                 bOK = false;
                                 break;
                             }