Few minor bugs

Hi,

While investigating some slow issue with re2j compared to java regular expression, I found a few issues that I am planning to create a PR for:

See these 2 commits I have in my branch (don't worry about the System.out.print lines):
1. https://github.com/julesbond007/re2j/commit/af6e399286975d972bfab90bc272e9ffbbcb876e
2. https://github.com/julesbond007/re2j/commit/3ea3aef9b4d46c57b5687308a12eac3db87a5997

Question:
`private void free(Thread t)` method is not used in `Machine.java` what was the intention?

Regards,

Jay