Make method static, fix possible NPE, and fix temp folder used

On [`HikariClassTransformer` line 358](https://github.com/brettwooldridge/HikariCP/blob/dev/core/src/main/java/com/zaxxer/hikari/javassist/HikariClassTransformer.java#L358), `destInitializer` is dereferenced, despite the fact that it may be null. Restructing the `if` fixes this. The other two changes are trivial. Let me know if you don't want one of these for whatever reason and I'll back it out.