Be more diligent in preventing drag line errors

They can happen even when there are no collisions, so we need to reset
the 'ok' flag before using the line again.

Fixes https://gitlab.com/kicad/code/kicad/issues/9555

pcbnew/router/pns_dragger.cpp

@@ -573,6 +573,9 @@ bool DRAGGER::dragWalkaround( const VECTOR2I& aP )
             ok = true;
         }
 
+        if( draggedWalk.CLine().PointCount() < 2 )
+            ok = false;
+
         if( ok )
         {
             PNS_DBG( Dbg(), AddLine, origLine.CLine(), BLUE, 50000, "drag-orig-line" );