do not overflow when computing delta (iowait can decrease - see proc(5))

[tkhattra]

cpubars gets SIGSEGV due to overflow when computing iowait delta

(gdb) p delta->cpus[0] $39 = {online = true, user = 0, nice = 0, sys = 0, iowait = 18446744073709551615, irq = 0, softirq = 1}

(gdb) p after->cpus[0] $51 = {online = true, user = 369655, nice = 612, sys = 384302, iowait = 48160, irq = 0, softirq = 798428}

(gdb) p before->cpus[0] $52 = {online = true, user = 369655, nice = 612, sys = 384302, iowait = 48161, irq = 0, softirq = 798427}