ktrace: Handle negative array sizes in ktrstructarray

ktrstructarray() may be used to create copies of kevent(2) change and
event arrays. It is called before parameter validation is done and so
should check for bogus array lengths before allocating a copy.

Reported by: syzkaller
Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30479