SHAPE_ARC: fixed polyline conversion bug introduced in e312e2b286

Fixes: lp:1826278
* https://bugs.launchpad.net/kicad/+bug/1826278

Fixes: lp:1826270
* https://bugs.launchpad.net/kicad/+bug/1826270

common/geometry/shape_arc.cpp

@@ -259,7 +259,7 @@ const SHAPE_LINE_CHAIN SHAPE_ARC::ConvertToPolyline( double aAccuracy ) const
         double a = sa;
 
         if( n != 0 )
-            sa += m_centralAngle * (double) i / (double) n;
+            a = m_centralAngle * (double) i / (double) n;
 
         double x = c.x + r * cos( a * M_PI / 180.0 );
         double y = c.y + r * sin( a * M_PI / 180.0 );