ex01: Fix warnings in sum.c, use slightly safer way of abusing type system.
ex01: Fix warnings in sum.c, use slightly safer way of abusing type
system.

vlstill/ttk4145,superboy0712/realtime,vlstill/ttk4145,superboy0712/realtime,superboy0712/realtime,vlstill/ttk4145,vlstill/ttk4145,superboy0712/realtime,superboy0712/realtime,vlstill/ttk4145,vlstill/ttk4145,superboy0712/realtime