Fixed segfault in connectivity algo for boards with orphaned nets

Fixes: lp:1701347
* https://bugs.launchpad.net/kicad/+bug/1701347

pcbnew/class_board.cpp

@@ -757,7 +757,11 @@ void BOARD::SetElementVisibility( GAL_LAYER_ID LAYER_aPCB, bool isEnabled )
         // so the hide/show option is a per item selection
 
         for( unsigned int net = 1; net < GetNetCount(); net++ )
-            GetConnectivity()->GetRatsnestForNet( net )->SetVisible( visible );
+        {
+            auto rn = GetConnectivity()->GetRatsnestForNet( net );
+            if( rn )
+                rn->SetVisible( visible );
+        }
 
         for( auto track : Tracks() )
             track->SetLocalRatsnestVisible( isEnabled );

pcbnew/connectivity.cpp

@@ -566,3 +566,12 @@ const std::vector<BOARD_CONNECTED_ITEM*> CONNECTIVITY_DATA::GetConnectedItems(
 
     return rv;
 }
+
+RN_NET* CONNECTIVITY_DATA::GetRatsnestForNet( int aNet )
+{
+    if ( aNet < 0 || aNet >= m_nets.size() )
+    {
+        return nullptr;
+    }
+    return m_nets[ aNet ];
+}

pcbnew/connectivity.h

@@ -122,11 +122,8 @@ public:
      * Function GetRatsnestForNet()
      * Returns the ratsnest, expressed as a set of graph edges for a given net.
      */
-    RN_NET* GetRatsnestForNet( int aNet )
-    {
-        return m_nets[aNet];
-    }
-
+    RN_NET* GetRatsnestForNet( int aNet );
+    
     /**
      * Function PropagateNets()
      * Propagates the net codes from the source pads to the tracks/vias.

pcbnew/ratsnest.cpp

@@ -103,6 +103,9 @@ void PCB_BASE_FRAME::DrawGeneralRatsnest( wxDC* aDC, int aNetcode )
     {
         RN_NET* net = connectivity->GetRatsnestForNet( i );
 
+        if( !net )
+            continue;
+
         if( ( aNetcode <= 0 ) || ( aNetcode == i ) )
         {
             for( const auto& edge : net->GetEdges() )

pcbnew/ratsnest_viewitem.cpp

@@ -86,6 +86,9 @@ void RATSNEST_VIEWITEM::ViewDraw( int aLayer, KIGFX::VIEW* aView ) const
     {
         RN_NET* net = m_data->GetRatsnestForNet( i );
 
+        if( !net )
+            continue;
+
         // Draw the "static" ratsnest
         if( i != highlightedNet )
             gal->SetStrokeColor( color );  // using the default ratsnest color for not highlighted