Empty content images list on specific situation

The [content images selection part](https://github.com/hwalsuklee/tensorflow-fast-style-transfer/blob/50438ad8b620af7d45ab8c4e050a7044d68ebe46/style_transfer_trainer.py#L22) should be modified. If ```len(content_images)``` is divided by ```batch_size```, then ```self.x_list``` will be empty, causing failure in training.