PNS router: fix a crash when switching to next/previous copper layer.

When (key + and - ) switching to next/previous layer before any usage of
the PNS router, Pcbnew crashed, likely due to incorrect value (perhaps
missing init) of the PNS_KICAD_IFACE (m_iface) m_board member.
So ensure this member is up to date before using it in ROUTER TOOL events.

pcbnew/router/router_tool.cpp

@@ -876,6 +876,10 @@ int ROUTER_TOOL::handleLayerSwitch( const TOOL_EVENT& aEvent, bool aForceVia )
     if( !IsToolActive() )
         return 0;
 
+    // Ensure PNS_KICAD_IFACE (m_iface) m_board member is up to date
+    // For some reason, this is not always the case
+    m_iface->SetBoard( board() );
+
     // First see if this is one of the switch layer commands
     BOARD*       brd           = board();
     LSET         enabledLayers = LSET::AllCuMask( brd->GetDesignSettings().GetCopperLayerCount() );