json_object_iter_set_new: Fix error branch leak.

This function needs to release a reference to value if the other
arguments are invalid.

Issue #135

src/value.c

@@ -258,7 +258,10 @@ json_t *json_object_iter_value(void *iter)
 int json_object_iter_set_new(json_t *json, void *iter, json_t *value)
 {
     if(!json_is_object(json) || !iter || !value)
+    {
+        json_decref(value);
         return -1;
+    }
 
     hashtable_iter_set(iter, value);
     return 0;