Fix: update() parent pointers not updated after recursive merge with JSON_DIAGNOSTICS (#5187)

* added fix for issue 4813

Signed-off-by: VasuBhakt <cpswastik31@gmail.com>

* added regression test for 4813

Signed-off-by: VasuBhakt <cpswastik31@gmail.com>

* moved test from unit-regression2 to unit-diagnostics

Signed-off-by: VasuBhakt <cpswastik31@gmail.com>

---------

Signed-off-by: VasuBhakt <cpswastik31@gmail.com>

include/nlohmann/json.hpp

@@ -3497,6 +3497,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                 if (it2 != m_data.m_value.object->end())
                 {
                     it2->second.update(it.value(), true);
+#if JSON_DIAGNOSTICS
+                    it2->second.set_parents();
+#endif
                     continue;
                 }
             }