fp_vpopen NULL deref on td->td_proc when called from pure thread context

Summary

fp_vpopen guards fsetcred with if(td->td_proc)(:175) but VOP_ACCESS(:165) and VOP_OPEN(:178) deref td->td_proc->p_ucred unconditionally. NULL deref panic if called from pure thread. Current callers (kern_checkpoint.c:522,641) in process context. Regression magnet.