examples/wrap: use modulus size for RSA NULL-pad test message

The RSA encrypt/decrypt test with TPM_ALG_NULL padding hardcoded
  message.size = 256, which is only valid for 2048-bit RSA keys.
  With TPM_ALG_NULL padding the TPM returns a full modulus-sized
  plaintext on decrypt, so with a 4096-bit key the 256-byte message
  never equals the 512-byte plaintext and the test reports
  TPM_RC_TESTING.

  Derive the message size from the key's own keyBits so the test
  passes for any modulus size.

  Reported against wolfTPM 4.0.0 on Nations NS350 hardware (NSING).

examples/wrap/wrap_test.c

@@ -338,7 +338,9 @@ int TPM2_Wrapper_TestArgs(void* userCtx, int argc, char *argv[])
     if (rc != 0) goto exit;
 
     /* Perform RSA encrypt / decrypt (no pad) */
-    message.size = 256; /* test message 0x11,0x11,etc */
+    /* With TPM_ALG_NULL padding, the TPM returns a full modulus-sized
+     * plaintext on decrypt, so the message must also be modulus-sized. */
+    message.size = rsaKey.pub.publicArea.parameters.rsaDetail.keyBits / 8;
     XMEMSET(message.buffer, 0x11, message.size);
     cipher.size = sizeof(cipher.buffer); /* encrypted data */
     rc = wolfTPM2_RsaEncrypt(&dev, &rsaKey, TPM_ALG_NULL,