Remove unneeded NULL checks in amd64's trap_fatal().

Since td_name is an array member of struct thread, it can never be NULL,
so the check can be removed. In addition, curproc can never be NULL,
so remove the if statement, and splice the two printfs() together.

While here, remove the u_long cast, and use the correct printf format
specifier curproc->p_pid.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D2695