Made JsonDocument's destructor protected (fixes #1480)

extras/tests/FailingBuilds/CMakeLists.txt

@@ -36,3 +36,6 @@ build_should_fail(read_long_long)
 add_executable(write_long_long write_long_long.cpp)
 set_property(TARGET write_long_long PROPERTY CXX_STANDARD 11)
 build_should_fail(write_long_long)
+
+add_executable(delete_jsondocument delete_jsondocument.cpp)
+build_should_fail(delete_jsondocument)

extras/tests/FailingBuilds/delete_jsondocument.cpp

@@ -0,0 +1,12 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2021
+// MIT License
+
+#include <ArduinoJson.h>
+
+struct Stream {};
+
+int main() {
+  JsonDocument* doc = new DynamicJsonDocument(42);
+  delete doc;
+}