Fix Windows build error

Signed-off-by: John Nunley <dev@notgull.net>

src/lib.rs

@@ -192,7 +192,7 @@ cfg_if::cfg_if! {
             fn register(&self, child: &std::process::Child) -> io::Result<()> {
                 // Called when a child exits.
                 unsafe extern "system" fn callback(_: *mut c_void, _: BOOLEAN) {
-                    Reaper::get().pipe.get().unwrap().sender.try_send(()).ok();
+                    Reaper::get().pipe.sender.try_send(()).ok();
                 }
 
                 // Register this child process to invoke `callback` on exit.