key: clear out secret data in DecodeExtKey

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).

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;
 }