Update thesis-uestc.bst

[liupeng3425]

解决一些计算机领域会议（例如ICLR）没有pages导致报错的问题。

@inproceedings{DBLP:conf/iclr/XuHLJ19,
  author       = {Keyulu Xu and
                  Weihua Hu and
                  Jure Leskovec and
                  Stefanie Jegelka},
  title        = {How Powerful are Graph Neural Networks?},
  booktitle    = {7th International Conference on Learning Representations, {ICLR} 2019,
                  New Orleans, LA, USA, May 6-9, 2019},
  publisher    = {OpenReview.net},
  year         = {2019},
  url          = {https://openreview.net/forum?id=ryGs6iA5Km},
  timestamp    = {Thu, 25 Jul 2019 13:03:15 +0200},
  biburl       = {https://dblp.org/rec/conf/iclr/XuHLJ19.bib},
  bibsource    = {dblp computer science bibliography, https://dblp.org}
}