CAD dock: fix a typo -> enable new snapping option

When X or Y is locked and user has mouse on top of a segment, CAD dock widget
will snap at the intersection of the segment and the axis.

src/gui/qgsadvanceddigitizingdockwidget.cpp

@@ -593,7 +593,7 @@ bool QgsAdvancedDigitizingDockWidget::applyConstraints( QgsMapMouseEvent *e )
     {
       point.setX( previousPt.x() + mXConstraint->value() );
     }
-    if ( !mSnappedSegment.isEmpty() && !mXConstraint->isLocked() )
+    if ( !mSnappedSegment.isEmpty() && !mYConstraint->isLocked() )
     {
       // intersect with snapped segment line at X ccordinate
       const double dx = mSnappedSegment.at( 1 ).x() - mSnappedSegment.at( 0 ).x();
@@ -620,7 +620,7 @@ bool QgsAdvancedDigitizingDockWidget::applyConstraints( QgsMapMouseEvent *e )
     {
       point.setY( previousPt.y() + mYConstraint->value() );
     }
-    if ( !mSnappedSegment.isEmpty() && !mYConstraint->isLocked() )
+    if ( !mSnappedSegment.isEmpty() && !mXConstraint->isLocked() )
     {
       // intersect with snapped segment line at Y ccordinate
       const double dy = mSnappedSegment.at( 1 ).y() - mSnappedSegment.at( 0 ).y();