change dps local var to a size_t

[danewalton]

The changed value is used below and casted to a size_t *, which was making it larger than a uint32_t on 64 bit systems. This caused memory corruption. https://github.com/Azure/azure-iot-middleware-freertos/blob/35dd34ea98b209e330365dd8c1f83b5908ebf51f/source/azure_iot_provisioning_client.c#L317