Fix UnblindConfidentialPair: use IsFullyValid() for ephemeral_key

if nNonce.vchCommitment contains invalid key of correct length,
IsValid() check will pass, but then assertion will be hit
when secp256k1_ec_pubkey_parse() is called on it inside CKey::ECDH().
Therefore, the check should be done with IsFullyValid()

src/blind.cpp

@@ -41,7 +41,7 @@ bool UnblindConfidentialPair(const CKey &key, const CConfidentialValue& confValu
         return false;
     }
     CPubKey ephemeral_key(nNonce.vchCommitment);
-    if (nNonce.vchCommitment.size() > 0 && !ephemeral_key.IsValid()) {
+    if (nNonce.vchCommitment.size() > 0 && !ephemeral_key.IsFullyValid()) {
         return false;
     }