Fix held_by_thread in no-std to return instead of panicing

Our `no-std` locks simply panic if a lock cannot be taken as there
should be no lock contention in a single-threaded environment.
However, the `held_by_thread` debug methods were delegating to the
lock methods which resulted in a panic when asserting that a lock
*is* held by the current thread.

Instead, they are updated here to call the relevant `RefCell`
testing methods.

Author: TheBlueMatt

lightning/src/sync/nostd_sync.rs

@@ -49,7 +49,7 @@ impl<T> Mutex<T> {
 impl<'a, T: 'a> LockTestExt<'a> for Mutex<T> {
 	#[inline]
 	fn held_by_thread(&self) -> LockHeldState {
-		if self.lock().is_err() { return LockHeldState::HeldByThread; }
+		if self.inner.try_borrow_mut().is_err() { return LockHeldState::HeldByThread; }
 		else { return LockHeldState::NotHeldByThread; }
 	}
 	type ExclLock = MutexGuard<'a, T>;
@@ -115,7 +115,7 @@ impl<T> RwLock<T> {
 impl<'a, T: 'a> LockTestExt<'a> for RwLock<T> {
 	#[inline]
 	fn held_by_thread(&self) -> LockHeldState {
-		if self.write().is_err() { return LockHeldState::HeldByThread; }
+		if self.inner.try_borrow_mut().is_err() { return LockHeldState::HeldByThread; }
 		else { return LockHeldState::NotHeldByThread; }
 	}
 	type ExclLock = RwLockWriteGuard<'a, T>;