Fix stack buffer overflow in MACThenDecrypt via block-alignment check

Add validation that ciphertext length is a multiple of CIPHER_BLOCK_SIZE
before calling decrypt(). The decrypt() function processes data in 16-byte
AES blocks, rounding up non-aligned input, which can write up to 8 bytes
past the caller's stack buffer. This can be triggered by crafted packets
that pass the 2-byte HMAC check (1-in-65536 chance per attempt), enabling
a persistent denial-of-service attack against repeaters and other nodes.

src/Utils.cpp

@@ -82,7 +82,9 @@ int Utils::MACThenDecrypt(const uint8_t* shared_secret, uint8_t* dest, const uin
     sha.finalizeHMAC(shared_secret, PUB_KEY_SIZE, hmac, CIPHER_MAC_SIZE);
   }
   if (memcmp(hmac, src, CIPHER_MAC_SIZE) == 0) {
-    return decrypt(shared_secret, dest, src + CIPHER_MAC_SIZE, src_len - CIPHER_MAC_SIZE);
+    int enc_len = src_len - CIPHER_MAC_SIZE;
+    if (enc_len % CIPHER_BLOCK_SIZE != 0) return 0;  // reject non-block-aligned ciphertext
+    return decrypt(shared_secret, dest, src + CIPHER_MAC_SIZE, enc_len);
   }
   return 0; // invalid HMAC
 }