Avoid returning a NULL pointer from the Intel hw PRNG ifunc resolver.

DTrace expects kernel function symbols of a non-zero size to have an
implementation, which is a reasonable invariant to preserve.

Reported and tested by: ler
Reviewed by: cem, kib
Approved by: so (delphij)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20218