benchmark timeout when benchtime set too long

[lkxiaolou]

When benchmark function execute time is very short, set benchtime=5s / 10s / 20s is ok.

BenchmarkNewObject-2            1000000000               0.279 ns/op

But set benchtime=120s will execute time out.

*** Test killed with quit: ran too long (11m0s).

1. benchtime short
   The B.n maybe 1 , 100 , 10000, 1000000, 100000000, 100000000 and exist.

2. benchtime long
   But set benchtime=120s, n = goalns * prevIters / prevns may be overflow(< 0) int64, and the B.n maybe 1, 100, 10000, 1000000, 100000000, 100000001, 100000002 .... and timeout.