[librsvg: 20/38] Extract function to get the top viewbox

[Federico Mena Quintero <federico src gnome org>]

commit 5524325c0836221443aa120648d6571ed2e7fd21
Author: Federico Mena Quintero <federico gnome org>
Date:   Tue Sep 22 12:58:21 2020 -0500

    Extract function to get the top viewbox

 rsvg_internals/src/drawing_ctx.rs | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)
---
diff --git a/rsvg_internals/src/drawing_ctx.rs b/rsvg_internals/src/drawing_ctx.rs
index 05548d85..73bbb4b8 100644
--- a/rsvg_internals/src/drawing_ctx.rs
+++ b/rsvg_internals/src/drawing_ctx.rs
@@ -300,11 +300,16 @@ impl DrawingCtx {
         )?)
     }
 
+    fn get_top_viewbox(&self) -> ViewBox {
+        let view_box_stack = self.view_box_stack.borrow();
+        *view_box_stack
+            .last()
+            .expect("view_box_stack must never be empty!")
+    }
+
     /// Gets the viewport that was last pushed with `push_view_box()`.
     pub fn get_view_params(&self) -> ViewParams {
-        let view_box_stack = self.view_box_stack.borrow();
-        let last = view_box_stack.len() - 1;
-        let vbox = view_box_stack[last];
+        let vbox = self.get_top_viewbox();
 
         ViewParams {
             dpi: self.dpi,