Illumos 6527 - Possible access beyond end of string in zpool comment

Illumos 6527 -  Possible access beyond end of string in zpool comment

6527 Possible access beyond end of string in zpool comment
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Gordon Ross <gwr@nexenta.com>

References:
  https://www.illumos.org/issues/6527
  https://github.com/illumos/illumos-gate/commit/2bd7a8d

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <tuxoko@gmail.com>