util: Don't allow DecodeBase64(...) of strings with embedded NUL characters

src/util/strencodings.cpp

@@ -191,6 +191,12 @@ std::vector<unsigned char> DecodeBase64(const char* p, bool* pf_invalid)
 
 std::string DecodeBase64(const std::string& str, bool* pf_invalid)
 {
+    if (!ValidAsCString(str)) {
+        if (pf_invalid) {
+            *pf_invalid = true;
+        }
+        return {};
+    }
     std::vector<unsigned char> vchRet = DecodeBase64(str.c_str(), pf_invalid);
     return std::string((const char*)vchRet.data(), vchRet.size());
 }