Replace unneeded unsafe calls to .get() with calls to .get_mut()

library/std/src/sync/mutex.rs

@@ -406,9 +406,7 @@ impl<T: ?Sized> Mutex<T> {
     /// ```
     #[stable(feature = "mutex_get_mut", since = "1.6.0")]
     pub fn get_mut(&mut self) -> LockResult<&mut T> {
-        // We know statically that there are no other references to `self`, so
-        // there's no need to lock the inner mutex.
-        let data = unsafe { &mut *self.data.get() };
+        let data = self.data.get_mut();
         poison::map_result(self.poison.borrow(), |_| data)
     }
 }