MFV r301526:
7035 string-related subroutines should validate input earlier

Reviewed by: Alex Wilson <alex.wilson@joyent.com>
Reviewed by: Bryan Cantrill <bryan@joyent.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>
Author: Patrick Mooney <pmooney@pfmooney.com>

illumos/illumos-gate@771e39c3b1d6e2e0ba230442d782d83c60098296

MFC after: 2 weeks