gstat: Set errno = 0 before strtoul

[cperciva]

Summary

The strtoul function sets errno on error, but does not clear it on
success; when using strtoul and checking errno (as one should) for
ERANGE / EINVAL afterwards, it's important to zero errno first.

While here, remove a dead store.

Fixes: 4fe8c1b67be0 ("Add error and range checking ... ")