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

(cherry picked from commit 833ce19221)

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" );