Illumos 6659 - nvlist_free(NULL) is a no-op

Illumos 6659 - nvlist_free(NULL) is a no-op

6659 nvlist_free(NULL) is a no-op
Reviewed by: Toomas Soome <tsoome@me.com>
Reviewed by: Marcel Telka <marcel@telka.sk>
Approved by: Robert Mustacchi <rm@joyent.com>

References:
  https://www.illumos.org/issues/6659
  https://github.com/illumos/illumos-gate/commit/aab83bb

Ported-by: David Quigley <dpquigl@davequigley.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4566