[librsvg/rustification] rsvg-cairo-render: Remove obsolete comment; add some assertions

[Federico Mena Quintero <federico src gnome org>]

commit 142a59969a98b9b628e86f04482ee4ff6f1b5411
Author: Federico Mena Quintero <federico gnome org>
Date:   Wed Nov 9 12:53:42 2016 -0600

    rsvg-cairo-render: Remove obsolete comment; add some assertions
    
    By the time rsvg_cairo_render_free() is called, all of its stacks
    should have already been emptied.

 rsvg-cairo-render.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/rsvg-cairo-render.c b/rsvg-cairo-render.c
index fdb4d74..f75d331 100644
--- a/rsvg-cairo-render.c
+++ b/rsvg-cairo-render.c
@@ -44,7 +44,9 @@ rsvg_cairo_render_free (RsvgRender * self)
 {
     RsvgCairoRender *me = RSVG_CAIRO_RENDER (self);
 
-    /* TODO */
+    g_assert (me->cr_stack == NULL);
+    g_assert (me->bb_stack == NULL);
+    g_assert (me->surfaces_stack == NULL);
 
     g_free (me);
 }