router: fixed use-after-free error occuring when a newly routed segment covered exactly an older one

Fixes: lp:1747973
* https://bugs.launchpad.net/kicad/+bug/1747973

pcbnew/router/pns_line_placer.cpp

@@ -1006,22 +1006,25 @@ bool LINE_PLACER::FixRoute( const VECTOR2I& aP, ITEM* aEndItem )
     else
         lastV = std::max( 1, l.SegmentCount() - 1 );
 
-    SEGMENT* lastSeg = NULL;
+    SEGMENT* lastSeg = nullptr;
 
     for( int i = 0; i < lastV; i++ )
     {
         const SEG& s = pl.CSegment( i );
-        std::unique_ptr< SEGMENT > seg( new SEGMENT( s, m_currentNet ) );
+        lastSeg = new SEGMENT( s, m_currentNet );
+        std::unique_ptr< SEGMENT > seg( lastSeg );
         seg->SetWidth( pl.Width() );
         seg->SetLayer( m_currentLayer );
-        lastSeg = seg.get();
-        m_lastNode->Add( std::move( seg ) );
+        if( ! m_lastNode->Add( std::move( seg ) ) )
+        {
+            lastSeg = nullptr;
+        }
     }
 
     if( pl.EndsWithVia() )
         m_lastNode->Add( Clone( pl.Via() ) );
 
-    if( realEnd )
+    if( realEnd && lastSeg )
         simplifyNewLine( m_lastNode, lastSeg );
 
     Router()->CommitRouting( m_lastNode );

pcbnew/router/pns_node.cpp

@@ -591,19 +591,21 @@ void NODE::addSegment( SEGMENT* aSeg )
     m_index->Add( aSeg );
 }
 
-void NODE::Add( std::unique_ptr< SEGMENT > aSegment, bool aAllowRedundant )
+bool NODE::Add( std::unique_ptr< SEGMENT > aSegment, bool aAllowRedundant )
 {
     if( aSegment->Seg().A == aSegment->Seg().B )
     {
         wxLogTrace( "PNS", "attempting to add a segment with same end coordinates, ignoring." );
-        return;
+        return false;
     }
 
     if( !aAllowRedundant && findRedundantSegment( aSegment.get() ) )
-        return;
+        return false;
 
     aSegment->SetOwner( this );
     addSegment( aSegment.release() );
+
+    return true;
 }
 
 void NODE::Add( std::unique_ptr< ITEM > aItem, bool aAllowRedundant )

pcbnew/router/pns_node.h

@@ -272,9 +272,10 @@ public:
      * Adds an item to the current node.
      * @param aSegment item to add
      * @param aAllowRedundant if true, duplicate items are allowed (e.g. a segment or via
+     * @return true if added
      * at the same coordinates as an existing one)
      */
-    void Add( std::unique_ptr< SEGMENT > aSegment, bool aAllowRedundant = false );
+    bool Add( std::unique_ptr< SEGMENT > aSegment, bool aAllowRedundant = false );
     void Add( std::unique_ptr< SOLID >   aSolid );
     void Add( std::unique_ptr< VIA >     aVia );