sysvsem: Fix a typo

Per jamie@ rpr can be NULL if the jail is created with sysvsem=disable.
But at least it doesn't appear to be fatal, since rpr is never dereferenced
but is only compared to other prison pointers.

Reviewed by: jamie
Differential revision: https://reviews.freebsd.org/D35198
MFC after: 2 weeks

(cherry picked from commit cb2ae6163174b90e999326ecec3699ee093a5d43)