Fix cgroup.events error when cgroup is gone before reading events

If cgroup is gone before reading the cgroup.events file then it will return
an error. Treat errors as unpopulated to account for it.

src/runc/container.rs

@@ -28,10 +28,15 @@ impl CgroupEventNotifier {
 
     fn populated(&mut self) -> Result<bool> {
         let file = self.file.get_mut();
-        file.seek(std::io::SeekFrom::Start(0))
-            .context("Cannot seek to start")?;
+        let Ok(_) = file.seek(std::io::SeekFrom::Start(0)) else {
+            return Ok(false);
+        };
         for line in BufReader::new(file).lines() {
-            let line = line.context("Cannot read line")?;
+            let Ok(line) = line else {
+                // IO errors on cgroup.events file indicate that the cgroup has been deleted, so
+                // it is no longer populated.
+                return Ok(false);
+            };
             if line.starts_with("populated ") {
                 return Ok(line.ends_with('1'));
             }