Fix simplify routine for last segment

The last segment of a track could be removed when the iterator does not
correctly loop.  Keeping the ll within bounds and checking for
equivalency instead of comparison works for last point

libs/kimath/src/geometry/shape_line_chain.cpp

@@ -2418,7 +2418,7 @@ void SHAPE_LINE_CHAIN::Simplify( int aMaxError )
         {
             bool too_far = false;
 
-            for( size_t ll = ii + 1; ll < kk; ++ll )
+            for( size_t ll = ( ii + 1 ) % m_points.size(); ll != kk; ++ll )
             {
                 if( !TestSegmentHitFast( m_points[ll], m_points[ii], m_points[kk], aMaxError ) )
                 {

qa/tests/libs/kimath/geometry/test_shape_line_chain.cpp

@@ -387,6 +387,27 @@ BOOST_AUTO_TEST_CASE( SimplifyDuplicatePoint )
 }
 
 
+// Test that duplicate point gets removed when we call simplify
+BOOST_AUTO_TEST_CASE( SimplifyKeepEndPoint )
+{
+    SHAPE_LINE_CHAIN chain;
+
+    chain.Append( { 114772424, 90949410 } );
+    chain.Append( { 114767360, 90947240 } );
+    chain.Append( { 114772429, 90947228 } );
+    chain.SetClosed( true );
+
+    BOOST_CHECK( GEOM_TEST::IsOutlineValid( chain ) );
+    BOOST_CHECK_EQUAL( chain.PointCount(), 3 );
+
+    chain.Simplify();
+
+    BOOST_CHECK_EQUAL( chain.CPoints().size(), chain.CShapes().size() );
+    BOOST_CHECK_EQUAL( chain.PointCount(), 3 );
+    BOOST_CHECK( GEOM_TEST::IsOutlineValid( chain ) );
+}
+
+
 struct REMOVE_SHAPE_CASE
 {
     std::string m_ctx_name;