Quoting the forum thread:
"When drawing a polyline with a label at the end, out of bounds occurs in function ClipPainterPrivate::labelPosition (file ClipPainter.cpp)."

Proposed fix:
void ClipPainterPrivate::labelPosition( const QPolygonF & polygon, QVector<QPointF>& labelNodes, 
                                        LabelPositionFlags labelPositionFlags)
{
    int labelPosition = 0;

    <...>

    if ( polygon.size() > 1 && labelPositionFlags.testFlag( LineEnd ) ) {
        if ( pointAllowsLabel( polygon.at( polygon.size() - 1 ) ) ) {
            labelNodes << polygon.at( polygon.size() - 1 );
        }

///# Error occurs at first iteration
///-        // The Label at the start of the polyline:
///-        for ( int it = polygon.size() - 1; it > 1; --it ) {
///# Instead use this:
        // The Label at the end of the polyline: 
        for ( int it = polygon.size() - 2; it > 0; --it ) {
///# End of fix.
            currentAllowsLabel = pointAllowsLabel( polygon.at( it ) );

            if ( currentAllowsLabel ) {
                QPointF node = interpolateLabelPoint( polygon.at( it + 1 ), polygon.at( it ),
                                                    labelPositionFlags );
                if ( node != QPointF( -1.0, -1.0 ) ) {
                    labelNodes << node;
                }
                break;
            }
        }
    }
}