Merge bitcoin#14242: Avoid triggering undefined behaviour (std::memse…

…t(nullptr, 0, 0)) if an invalid string is passed to DecodeSecret(...)

d855e4c Avoid triggering undefined behaviour (std::memset(nullptr, 0, 0)) if an invalid string is passed to DecodeSecret(...) (practicalswift)

Pull request description:

  Avoid triggering undefined behaviour (`std::memset(nullptr, 0, 0)`) if an invalid string is passed to `DecodeSecret(...)`.

  Background reading: [memcpy (and friends) with NULL pointers](https://www.imperialviolet.org/2016/06/26/nonnull.html)

  Steps to reproduce:

  ```
  ./configure --with-sanitizers=undefined && make check && ./test/functional/test_runner.py
  ```

Tree-SHA512: b8325ced4f724d9c03065e0747af56b1f297a90d9fb09a24d46c3231a90dce3df6299f2c41f863b5cec18eaeded7b46ee4b93d9a52adc2541eb4c44d2c0965d9

src/key_io.cpp

@@ -142,7 +142,9 @@ std::string EncodeExtKey(const CExtKey& key)
     data.resize(size + BIP32_EXTKEY_SIZE);
     key.Encode(data.data() + size);
     std::string ret = EncodeBase58Check(data);
-    memory_cleanse(data.data(), data.size());
+    if (!data.empty()) {
+        memory_cleanse(data.data(), data.size());
+    }
     return ret;
 }