Rewrite json_object_object_add to replace just the value if the key already exists so keys remain valid.

This is particularly useful when replacing values in a loop, since it allows
 the key used by json_object_object_foreach to continue to be used.

tests/testReplaceExisting.c

@@ -0,0 +1,56 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stddef.h>
+#include <string.h>
+
+#include "json.h"
+
+int main(int argc, char **argv)
+{
+	MC_SET_DEBUG(1);
+
+	/*
+	 * Check that replacing an existing object keeps the key valid,
+	 * and that it keeps the order the same.
+	 */
+	json_object *my_object = json_object_new_object();
+	json_object_object_add(my_object, "foo1", json_object_new_string("bar1"));
+	json_object_object_add(my_object, "foo2", json_object_new_string("bar2"));
+	json_object_object_add(my_object, "foo3", json_object_new_string("bar3"));
+	const char *original_key = NULL;
+	int orig_count = 0;
+	json_object_object_foreach(my_object, key, val)
+	{
+		printf("Key at index %d is [%s]\n", orig_count, key);
+		orig_count++;
+		if (strcmp(key, "foo2") != 0)
+			continue;
+		printf("replacing value for key [%s]\n", key);
+		original_key = key;
+		json_object_object_add(my_object, key, json_object_new_string("zzz"));
+	}
+
+	printf("==== second loop starting ====\n");
+
+	int new_count = 0;
+	int retval = 0;
+	json_object_object_foreach(my_object, key2, val2)
+	{
+		printf("Key at index %d is [%s]\n", new_count, key2);
+		new_count++;
+		if (strcmp(key2, "foo2") != 0)
+			continue;
+		printf("pointer for key [%s] does %smatch\n", key2,
+		       (key2 == original_key) ? "" : "NOT ");
+		if (key2 != original_key)
+			retval = 1;
+	}
+	if (new_count != orig_count)
+	{
+		printf("mismatch between original count (%d) and new count (%d)\n",
+		       orig_count, new_count);
+		retval = 1;
+	}
+
+	return 0;
+}