Fix assertion poolIndex < count_ after JsonDocument::clear()

Fixes #2034

extras/tests/JsonDocument/clear.cpp

@@ -0,0 +1,47 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2024, Benoit BLANCHON
+// MIT License
+
+#include <ArduinoJson.h>
+#include <catch.hpp>
+
+#include <stdlib.h>  // malloc, free
+#include <string>
+
+#include "Allocators.hpp"
+
+TEST_CASE("JsonDocument::clear()") {
+  SpyingAllocator spy;
+  JsonDocument doc(&spy);
+
+  SECTION("null") {
+    doc.clear();
+
+    REQUIRE(doc.isNull());
+    REQUIRE(spy.log() == AllocatorLog{});
+  }
+
+  SECTION("releases resources") {
+    doc[std::string("hello")] = std::string("world");
+    spy.clearLog();
+
+    doc.clear();
+
+    REQUIRE(doc.isNull());
+    REQUIRE(spy.log() == AllocatorLog{
+                             Deallocate(sizeofPool()),
+                             Deallocate(sizeofString("hello")),
+                             Deallocate(sizeofString("world")),
+                         });
+  }
+
+  SECTION("clear free list") {  // issue #2034
+    JsonObject obj = doc.to<JsonObject>();
+    obj["a"] = 1;
+    obj.clear();  // puts the slot in the free list
+
+    doc.clear();
+
+    doc["b"] = 2;  // will it pick from the free list?
+  }
+}