make borrow checker happy

Signed-off-by: tison <[email protected]>

Author: tisonkun

library/core/src/cell/once.rs

@@ -219,10 +219,9 @@ impl<T> OnceCell<T> {
     where
         F: FnOnce() -> Result<T, E>,
     {
-        if let Some(val) = self.get_mut() {
-            return Ok(val);
+        if self.get().is_none() {
+            self.try_init(f)?;
         }
-        self.try_init(f)?;
         Ok(self.get_mut().unwrap())
     }