libjail: Guard against programmer error in jailparam_export()

If the caller didn't use jailparam_import() to fetch the parameter
value, an attempt to export it will trigger a segfault. Make it a bit
easier to figure out what's happening in this situation.

PR: 276809
Reviewed by: jamie
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43732

(cherry picked from commit dfabf3efaa795e657f3858b7e2d0b11f8a51ecb5)