ng_parse: disallow negative length for malloc

This is an interim robustness improvement; further improvements as
described in the PR and/or Phabricator review are still needed.

PR: 267334
Reported by: Robert Morris <rtm@lcs.mit.edu>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37229

(cherry picked from commit ae4f39464c61182c2bdfd3cc61594f8707b3daf0)