Fix leak introduced in inner navigation refactoring

A inner-navigate event can override an existing pending queued navigation. When
it does, the previously queued navigation has to be cleaned up. We were doing
this, but it must have been stripped out when navigation was refactored to work
with frames.

src/browser/Page.zig

@@ -652,6 +652,10 @@ fn scheduleNavigationWithArena(originator: *Page, arena: Allocator, request_url:
         .navigation_type = std.meta.activeTag(nt),
     };
 
+    if (target._queued_navigation) |existing| {
+        target.arena_pool.release(existing.arena);
+    }
+
     target._queued_navigation = qn;
     return session.scheduleNavigation(target);
 }