Round-rect pads: use a circle to draw degenerated shapes.

A degenerated shape is a round-rect pads with same X and Y size, and a
radius very near 50% of the size.

Fixes #5768
https://gitlab.com/kicad/code/kicad/issues/5768

pcbnew/class_pad.cpp

@@ -347,7 +347,19 @@ void D_PAD::BuildEffectiveShapes( PCB_LAYER_ID aLayer ) const
         wxSize  trap_delta( 0, 0 );
 
         if( effectiveShape == PAD_SHAPE_ROUNDRECT )
+        {
             half_size -= wxPoint( r, r );
+
+            // Avoid degenerated shapes (0 length segments) that always create issues
+            // For roundrect pad very near a circle, use only a circle
+            const int min_len = Millimeter2iu( 0.0001);
+
+            if( half_size.x < min_len && half_size.y < min_len )
+            {
+                add( new SHAPE_CIRCLE( shapePos, r ) );
+                break;
+            }
+        }
         else if( effectiveShape == PAD_SHAPE_TRAPEZOID )
             trap_delta = m_deltaSize / 2;