Fix memory leak in zpool_clear_label()

Clang's static analyzer reported a memory leak in zpool_clear_label().
Upon review, it turns out to be right. This should be a very short lived
leak because no daemons use this functionality, but that does not
preclude the possibility of third party daemons that do use it. Lets fix
it to be a good Samaritan.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2330
Remove superfluous statement

Clang's static analyzer reported that the value assigned to pcksum is
never used. That is because we initialize both zc and pcksum to {{ 0 }}
and then do `pcksum = zc;`. That is fairly pointless. However, it has
the effect of generating a false positive in Clang's static analyzer.
Since noise from false positives can obscure real issues, we fix it
anyway.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2330