Clarify intention that option is Some in code

The debug_assert adds a stronger signal that the option must be Some when it is being constructed.

libherokubuildpack/src/output/background.rs

@@ -108,10 +108,13 @@ pub(crate) mod state {
     {
         /// Preserve internal state by ensuring the `Option` is always populated
         pub(crate) fn new(join_handle: JoinHandle<W>, sender: Sender<()>) -> Self {
-            PrintGuard {
+            let guard = PrintGuard {
                 join_handle: Some(join_handle),
                 stop_signal: sender,
-            }
+            };
+            debug_assert!(guard.join_handle.is_some());
+
+            guard
         }
 
         /// The only thing a consumer can do is stop the dots and receive