Add sanity check after key generation

Add a sanity check to prevent cosmic rays from flipping a bit in the
generated public key, or bugs in the elliptic curve code. This is
simply done by signing a (randomized) message, and verifying the
result.

src/wallet.cpp

@@ -79,6 +79,7 @@ CPubKey CWallet::GenerateNewKey()
         SetMinVersion(FEATURE_COMPRPUBKEY);
 
     CPubKey pubkey = secret.GetPubKey();
+    assert(secret.VerifyPubKey(pubkey));
 
     // Create new metadata
     int64_t nCreationTime = GetTime();