Merge bitcoin/bitcoin#31166: key: clear out secret data in DecodeExtKey

559a8dd9c0 key: clear out secret data in `DecodeExtKey` (Sebastian Falbesoner)

Pull request description:

  Same as in `DecodeSecret`, we should also clear out the secret data from the vector resulting from the Base58Check parsing for xprv keys. Note that the if condition is needed in order to avoid UB, see #14242 (commit d855e4cac8).

ACKs for top commit:
  davidgumberg:
    utACK 559a8dd9c0
  achow101:
    ACK 559a8dd9c0
  tdb3:
    cr ACK 559a8dd9c0
  laanwj:
    Code review ACK 559a8dd9c0

Tree-SHA512: c22499fe2899a9a5a58159ec55e94cf961570d8af06358d4a6d1943d567be9b88657af90d060d3083985ea957886a4f91bb762a2fcf3311007e7a535b42b0fde

src/key_io.cpp

@@ -274,6 +274,9 @@ CExtKey DecodeExtKey(const std::string& str)
             key.Decode(data.data() + prefix.size());
         }
     }
+    if (!data.empty()) {
+        memory_cleanse(data.data(), data.size());
+    }
     return key;
 }