kvm_geterr: handle kd == NULL in a deterministic/graceful manner

Return a NUL string instead of just working by accident with kvm_geterr(3)
when MALLOC_PRODUCTION is disabled (I didn't confirm the MALLOC_PRODUCTION
being enabled path).

Document the new explicit return behavior for kvm_geterr(3), as well
as the previous implicit behavior, i.e., the buffer attached to
returned via kvm_geterr(3) would be empty if a previous error hadn't been
stored in kd.

Differential Revision: D10022
MFC after: 1 week
Reviewed by: vangyzen
Sponsored by: Dell EMC Isilon