PNS: Fix missing via on zero-length

The current node needs to be kept for zero-length lines when placing a
via.

Fixes https://gitlab.com/kicad/code/kicad/issues/4005

pcbnew/router/pns_line_placer.cpp

@@ -1107,7 +1107,6 @@ bool LINE_PLACER::FixRoute( const VECTOR2I& aP, ITEM* aEndItem, bool aForceFinis
             return false;
 
         m_lastNode->Add( Clone( pl.Via() ) );
-        m_lastNode = NULL;
         m_currentNode = NULL;
 
         m_idle = true;