Fix kvm_getprocs(3) error reporting in ps(1).

Previously it just didn't work at all - kvm_getprocs(3) doesn't update
the &nentries when it returns NULL. The end result was that ps(1) showed
garbage data instead of reporting kinfo_proc size mismatch.

Reviewed by: cem
Obtained from: CheriBSD
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D12414