Return 0 instead of -1 on null pointer (ARMmbed#1728)

Author: verkkila

source/6LoWPAN/Thread/thread_test_api.c

@@ -625,7 +625,8 @@ int thread_test_version_set(int8_t interface_id, uint8_t version)
     thread_version = version;
     cur = protocol_stack_interface_info_get_by_id(interface_id);
     if (!cur) {
-        return -1;
+        /*We already stored the new Thread version above, so even if cur is NULL the version is updated.*/
+        return 0;
     }
     cur->thread_info->version = version;
     return 0;