nem: Prevent underflow in NEM_DECRYPTED_SIZE

nem.h

@@ -55,7 +55,10 @@
 #define NEM_ENCRYPTED_SIZE(size)         (((size) + AES_BLOCK_SIZE) / AES_BLOCK_SIZE * AES_BLOCK_SIZE)
 #define NEM_ENCRYPTED_PAYLOAD_SIZE(size) (AES_BLOCK_SIZE + NEM_SALT_SIZE + NEM_ENCRYPTED_SIZE(size))
 
-#define NEM_DECRYPTED_SIZE(buffer, size)         ((size) - ((buffer)[(size) - 1]))
+#define _NEM_PADDING_SIZE(buffer, size)  ((buffer)[(size) - 1])
+#define NEM_PADDING_SIZE(buffer, size)   (_NEM_PADDING_SIZE(buffer, size) > (size) ? (size) : _NEM_PADDING_SIZE(buffer, size))
+
+#define NEM_DECRYPTED_SIZE(buffer, size) ((size) - NEM_PADDING_SIZE(buffer, size))
 
 typedef struct {
 	ed25519_public_key public_key;