Adding call to check(err) after os.Open

Clearly we were just able to read the content via ioutil.ReadFile but for consistency I think it would be better to call check(err) after the call to os.Open("/tmp/dat") on line 34.

Tools like errcheck (https://github.com/kisielk/errcheck) will complain about uncaught / unhandled err's. Also can we really be sure some 3rd party process didn't nuke '/tmp' during execution :-)