fix deref null Steps

stepmania/src/EditMenu.cpp

@@ -362,7 +362,7 @@ void EditMenu::OnRowValueChanged( EditMenuRow row )
 			else
 			{
 				Steps *pSteps = GetSelectedSong()->GetStepsByDifficulty( GetSelectedStepsType(), dc );
-				if( UNLOCKMAN->StepsIsLocked( GetSelectedSong(), pSteps ) )
+				if( pSteps && UNLOCKMAN->StepsIsLocked( GetSelectedSong(), pSteps ) )
 					pSteps = NULL;
 
 				switch( EDIT_MODE.GetValue() )

stepmania/src/UnlockManager.cpp

@@ -148,6 +148,7 @@ const UnlockEntry *UnlockManager::FindSong( const Song *pSong ) const
 
 const UnlockEntry *UnlockManager::FindSteps( const Song *pSong, const Steps *pSteps ) const
 {
+	ASSERT( pSong && pSteps );
 	FOREACH_CONST( UnlockEntry, m_UnlockEntries, e )
 		if( e->m_pSong == pSong  &&  e->m_dc == pSteps->GetDifficulty() )
 			return &(*e);