Barrier: Fix potential locking issue (#17944)

* Barrier: Fix potential locking issue

* add changelog

changelog/17944.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+core: Fix potential deadlock if barrier ciphertext is less than 4 bytes.
+```

vault/barrier_aes_gcm.go

@@ -1069,11 +1069,13 @@ func (b *AESGCMBarrier) Decrypt(_ context.Context, key string, ciphertext []byte
 	}
 
 	if len(ciphertext) == 0 {
+		b.l.RUnlock()
 		return nil, fmt.Errorf("empty ciphertext")
 	}
 
 	// Verify the term
 	if len(ciphertext) < 4 {
+		b.l.RUnlock()
 		return nil, fmt.Errorf("invalid ciphertext term")
 	}
 	term := binary.BigEndian.Uint32(ciphertext[:4])