[librsvg/librsvg-2.46] AcquiredNode: store the stack as an Option.

[Federico Mena Quintero <federico src gnome org>]

commit 098f83df30470ca9437c19f203b6fc0af239ec89
Author: Federico Mena Quintero <federico gnome org>
Date:   Tue Oct 15 08:35:54 2019 -0500

    AcquiredNode: store the stack as an Option.
    
    We'll move to a model where we only push nodes that are only
    accessible by reference, not all nodes that are acquired.

 rsvg_internals/src/drawing_ctx.rs | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)
---
diff --git a/rsvg_internals/src/drawing_ctx.rs b/rsvg_internals/src/drawing_ctx.rs
index a6ccc824..cc22d43e 100644
--- a/rsvg_internals/src/drawing_ctx.rs
+++ b/rsvg_internals/src/drawing_ctx.rs
@@ -1069,15 +1069,17 @@ impl From<RsvgRectangle> for cairo::Rectangle {
 }
 
 pub struct AcquiredNode {
-    stack: Rc<RefCell<NodeStack>>,
+    stack: Option<Rc<RefCell<NodeStack>>>,
     node: RsvgNode,
 }
 
 impl Drop for AcquiredNode {
     fn drop(&mut self) {
-        let mut stack = self.stack.borrow_mut();
-        let last = stack.pop().unwrap();
-        assert!(last == self.node);
+        if let Some(ref stack) = self.stack {
+            let mut stack = stack.borrow_mut();
+            let last = stack.pop().unwrap();
+            assert!(last == self.node);
+        }
     }
 }
 
@@ -1141,7 +1143,7 @@ impl AcquiredNodes {
         } else {
             self.node_stack.borrow_mut().push(&node);
             let acquired = AcquiredNode {
-                stack: self.node_stack.clone(),
+                stack: Some(self.node_stack.clone()),
                 node: node.clone()
             };
             Ok(acquired)