Add a missing statement in segments reservation.

Also adds an assert in FinishItem.

Fixes https://gitlab.com/kicad/code/kicad/issues/12448

common/gal/opengl/opengl_gal.cpp

@@ -2228,6 +2228,7 @@ void OPENGL_GAL::drawSegmentChain( const std::function<VECTOR2D( int )>& aPointG
         if( startx == endx || starty == endy )
         {
             vertices += 3; // One circle
+            continue;
         }
 
         if( m_isFillEnabled || aWidth == 1.0 )

common/gal/opengl/vertex_manager.cpp

@@ -167,6 +167,8 @@ void VERTEX_MANAGER::SetItem( VERTEX_ITEM& aItem ) const
 
 void VERTEX_MANAGER::FinishItem() const
 {
+    assert( m_reservedSpace == 0 && m_reserved == nullptr );
+
     m_container->FinishItem();
 }