remove redundant if-check, introduce Q_ASSERT to check sanity
remove redundant if-check, introduce Q_ASSERT to check sanity

tzapzoor/marble,quannt24/marble,tucnak/marble,tzapzoor/marble,tzapzoor/marble,probonopd/marble,probonopd/marble,quannt24/marble,tzapzoor/marble,probonopd/marble,probonopd/marble,tucnak/marble,quannt24/marble,probonopd/marble,quannt24/marble,tzapzoor/marble,probonopd/marble,tucnak/marble,tzapzoor/marble,quannt24/marble,tucnak/marble,probonopd/marble,quannt24/marble,tucnak/marble,quannt24/marble,tucnak/marble,tucnak/marble,tzapzoor/marble,tzapzoor/marble