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

559a8dd 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 d855e4c).

ACKs for top commit:
  davidgumberg:
    utACK bitcoin/bitcoin@559a8dd
  achow101:
    ACK 559a8dd
  tdb3:
    cr ACK 559a8dd
  laanwj:
    Code review ACK 559a8dd

Tree-SHA512: c22499fe2899a9a5a58159ec55e94cf961570d8af06358d4a6d1943d567be9b88657af90d060d3083985ea957886a4f91bb762a2fcf3311007e7a535b42b0fde

Author: achow101

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;
 }