Fix useless error check in extractTarGz

[alexandear]

This PR fixes impossible condition nil != nil.

Consider the following code:

f, err := os.Create(path)
if err != nil {
	return err
}

f.Chmod(info.Mode())
if err != nil {
	return err
}

The second if err != nil effectively becomes if nil != nil due to the preceding if err != nil check.

There are two ways to fix this: the first is to simply delete the second if err != nil, and the second is to handle an error from f.Chmod, which is the option I have chosen.

[bentranter]

Great find @alexandear, thank you!