Fix obvious mistakes like spelling errors

Hi, after studying your project, I learn pretty much from your work, in particular visitor design pattern and the elegantly-implemented SQL parser.

While I am reading the code, I fix a lot of obvious minor errors, like typos and places potentially causing NullPointerException. I also removed a few redundant code. 

By the way, all unit tests are still passing after applying this commit. Wish my pull request is valuable to Druid project.