pcbnew Check for valid nodes before drawing ratsnest

We can invalidate a node by rotating.  Make sure that our endpoints for
the ratsnest are valid before attempting to draw.

Fixes: lp:1775727
* https://bugs.launchpad.net/kicad/+bug/1775727

pcbnew/ratsnest.cpp

@@ -115,6 +115,9 @@ void PCB_BASE_FRAME::DrawGeneralRatsnest( wxDC* aDC, int aNetcode )
                 auto sn = edge.GetSourceNode();
                 auto dn = edge.GetTargetNode();
 
+                if( !sn->Valid() || !dn->Valid() )
+                    continue;
+
                 bool enable = !sn->GetNoLine() && !dn->GetNoLine();
                 bool show = sn->Parent()->GetLocalRatsnestVisible()
                             || dn->Parent()->GetLocalRatsnestVisible();

pcbnew/ratsnest_viewitem.cpp

@@ -109,6 +109,9 @@ void RATSNEST_VIEWITEM::ViewDraw( int aLayer, KIGFX::VIEW* aView ) const
             const VECTOR2I source( sourceNode->Pos() );
             const VECTOR2I target( targetNode->Pos() );
 
+            if( !sourceNode->Valid() || !targetNode->Valid() )
+                continue;
+
             bool enable =  !sourceNode->GetNoLine() && !targetNode->GetNoLine();
             bool show = sourceNode->Parent()->GetLocalRatsnestVisible() || targetNode->Parent()->GetLocalRatsnestVisible();