Fix clang analizer warnings

I've run http://clang-analyzer.llvm.org/ on 'master' branch and I've found a couple of warnings on the code base.

Before this changes (running the check 'master') looks like this:

![master](https://cloud.githubusercontent.com/assets/9576/3269698/e49a197e-f2f2-11e3-8550-dbc42866b020.png)

After applying this changes (running the check on 'fix-clang-analizer-warnings') looks like this: 

![fix-clang-analizer-warnings](https://cloud.githubusercontent.com/assets/9576/3269708/1d665088-f2f3-11e3-990b-c3d1f56d56a2.png)

This is the output of './minirake test' after the changes:

mrbtest - Embeddable Ruby Test

......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
Total: 806
   OK: 806
   KO: 0
Crash: 0
 Time: 0.04303599999999 seconds

I've got rid of dead references and some divisions by 0 there are still some warnings but I'm checking for false positives.

NOTE: I'm not a C developer :)