Add assert to SHAPE_LINE_CHAIN::Segment + Deduplicate code

CSegment should have the exact same logic as Segment

Also, moving definition to .cpp file to reduce need to rebuild every time

libs/kimath/include/geometry/shape_line_chain.h

@@ -330,16 +330,7 @@ public:
      *        from the end (i.e. -1 means the last segment in the line chain).
      * @return a segment at the \a aIndex in the line chain.
      */
-    SEG Segment( int aIndex )
-    {
-        if( aIndex < 0 )
-            aIndex += SegmentCount();
-
-        if( aIndex == (int)( m_points.size() - 1 ) && m_closed )
-            return SEG( m_points[aIndex], m_points[0], aIndex );
-        else
-            return SEG( m_points[aIndex], m_points[aIndex + 1], aIndex );
-    }
+    SEG Segment( int aIndex ) const;
 
     /**
      * Return a constant copy of the \a aIndex segment in the line chain.
@@ -348,16 +339,7 @@ public:
      *        from the end (i.e. -1 means the last segment in the line chain).
      * @return a segment at \a aIndex in the line chain.
      */
-    const SEG CSegment( int aIndex ) const
-    {
-        if( aIndex < 0 )
-            aIndex += SegmentCount();
-
-        if( aIndex == (int)( m_points.size() - 1 ) && m_closed )
-            return SEG( m_points[aIndex], m_points[0], aIndex );
-        else
-            return SEG( m_points[aIndex], m_points[aIndex + 1], aIndex );
-    }
+    const SEG CSegment( int aIndex ) const { return Segment( aIndex ); }
 
     /**
      * Return the vertex index of the next shape in the chain, or -1 if \a aPointIndex is the

libs/kimath/src/geometry/shape_line_chain.cpp

@@ -1027,6 +1027,21 @@ int SHAPE_LINE_CHAIN::ShapeCount() const
 }
 
 
+SEG SHAPE_LINE_CHAIN::Segment( int aIndex ) const
+{
+    if( aIndex < 0 )
+        aIndex += SegmentCount();
+
+    wxCHECK( aIndex < SegmentCount() && aIndex >= 0,
+             m_points.size() > 0 ? SEG( m_points.back(), m_points.back() ) : SEG( 0, 0, 0, 0 ) );
+
+    if( aIndex == (int) ( m_points.size() - 1 ) && m_closed )
+        return SEG( m_points[aIndex], m_points[0], aIndex );
+    else
+        return SEG( m_points[aIndex], m_points[aIndex + 1], aIndex );
+}
+
+
 int SHAPE_LINE_CHAIN::NextShape( int aPointIndex ) const
 {
     if( aPointIndex < 0 )