Build Warnings

It seems in since we have the new file format some warnings were introduced into the build process:

```
/home/vagrant/mruby/src/load.c: In function 'read_rite_lineno_record':
/home/vagrant/mruby/src/load.c:231:7: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
/home/vagrant/mruby/src/load.c: In function 'read_rite_section_lineno_file':
/home/vagrant/mruby/src/load.c:400:8: warning: ignoring return value of 'fread', declared with attribute warn_unused_result [-Wunused-result]
/home/vagrant/mruby/src/load.c:409:10: warning: ignoring return value of 'fread', declared with attribute warn_unused_result [-Wunused-result]
/home/vagrant/mruby/src/load.c:413:10: warning: ignoring return value of 'fread', declared with attribute warn_unused_result [-Wunused-result]
/home/vagrant/mruby/src/load.c: In function 'mrb_read_irep':
/home/vagrant/mruby/src/load.c:284:9: warning: 'len' may be used uninitialized in this function [-Wuninitialized]
/home/vagrant/mruby/src/load.c:269:12: note: 'len' was declared here
```

With this pull request I init the uninitialized one and catch the return value from fread. I guess we should have a check if it makes sense to react on the return value in a meaningful manner.