Add an assertion that the pcb_nsaved field of the pcb be less than

MAXWIN to the register window manipulation functions - rwindow_load()
calls rwindow_save() so this one addition should take care of both.
This should help find places that pcb_nsaved doesn't get initialized
properly.

Suggested by:	jake

sys/sparc64/sparc64/rwindow.c

@@ -84,6 +84,8 @@ rwindow_save(struct thread *td)
 	    td->td_proc->p_comm, pcb->pcb_nsaved);
 
 	flushw();
+	KASSERT(pcb->pcb_nsaved < MAXWIN,
+	    ("rwindow_save: pcb_nsaved > MAXWIN"));
 	if ((i = pcb->pcb_nsaved) == 0)
 		return (0);
 	ausp = pcb->pcb_rwsp;