Fix r356919.

Instead of waiting for pc_curthread which is overwritten by
init_secondary_tail(), wait for non-NULL pc_curpcb, to be set by the
first context switch.
Assert that pc_curpcb is not set too early.

Reported and tested by: rlibby
Reviewed by: markj, rlibby
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D23330