Fix memory leak in Json move assignment operator

*  When move assignment is creates a cyclic ownership pattern
   memory was being leaked.  This patch breaks the cycle.

*  Fixes: #2572

src/ripple/json/impl/json_value.cpp

@@ -348,10 +348,10 @@ Value::~Value ()
 }
 
 Value&
-Value::operator= ( const Value& other )
+Value::operator=(Value const& other)
 {
-    Value temp ( other );
-    swap ( temp );
+    Value tmp(other);
+    swap(tmp);
     return *this;
 }
 
@@ -365,9 +365,10 @@ Value::Value ( Value&& other ) noexcept
 }
 
 Value&
-Value::operator= ( Value&& other ) noexcept
+Value::operator=(Value&& other)
 {
-    swap ( other );
+    Value tmp(std::move(other));
+    swap(tmp);
     return *this;
 }
 

src/ripple/json/json_value.h

@@ -233,10 +233,10 @@ public:
     Value ( const Value& other );
     ~Value ();
 
-    Value& operator= ( const Value& other );
+    Value& operator= ( Value const& other );
+    Value& operator= ( Value&& other );
 
     Value ( Value&& other ) noexcept;
-    Value& operator= ( Value&& other ) noexcept;
 
     /// Swap values.
     /// \note Currently, comments are intentionally not swapped, for

src/test/json/json_value_test.cpp

@@ -296,6 +296,17 @@ struct json_value_test : beast::unit_test::suite
         }
     }
 
+  void
+    test_leak()
+    {
+      // When run with the address sanitizer, this test confirms there is no
+      // memory leak with the scenario below.
+      Json::Value a;
+      a[0u] = 1;
+      a = std::move(a[0u]);
+      pass();
+    }
+
     void run () override
     {
         test_bool ();
@@ -306,6 +317,7 @@ struct json_value_test : beast::unit_test::suite
         test_comparisons ();
         test_compact ();
         test_nest_limits ();
+        test_leak();
     }
 };