Fix frame skip for standard library

[flisky]

it should be same with log.Output calldepth.

The test case covers wrong position -

go.uber.org/zap.(*Logger).check(0xc42038eb40, 0x0, 0xc4204ec330, 0xa, 0xc4204ec330)
	/home/flisky/.gvm/pkgsets/go1.8/global/src/go.uber.org/zap/logger.go:269 +0x62f
go.uber.org/zap.(*Logger).Info(0xc42038eb40, 0xc4204ec330, 0xa, 0x0, 0x0, 0x0)
	/home/flisky/.gvm/pkgsets/go1.8/global/src/go.uber.org/zap/logger.go:160 +0x44
go.uber.org/zap.(*loggerWriter).Write(0xc42049e3a0, 0xc4204ec300, 0xa, 0x10, 0xa, 0xc4204ec300, 0x0)
	/home/flisky/.gvm/pkgsets/go1.8/global/src/go.uber.org/zap/global.go:112 +0xbd
log.(*Logger).Output(0xc42028b360, 0x2, 0xc4204ec2f0, 0xa, 0x0, 0x0)
	/home/flisky/.gvm/gos/go1.8/src/log/log.go:168 +0x210
log.(*Logger).Print(0xc42028b360, 0xc42012ee10, 0x1, 0x1)
	/home/flisky/.gvm/gos/go1.8/src/log/log.go:180 +0x6a
go.uber.org/zap.TestNewStdLog.func1(0xc42038eae0, 0xc42010a000)
	/home/flisky/.gvm/pkgsets/go1.8/global/src/go.uber.org/zap/global_test.go:98 +0xed
go.uber.org/zap.withLogger(0x944060, 0xc420060000, 0x93d9a0, 0xc4204ec295, 0xc420203f88, 0x1, 0x1, 0xc420203f78)
	/home/flisky/.gvm/pkgsets/go1.8/global/src/go.uber.org/zap/common_test.go:41 +0x12b
go.uber.org/zap.TestNewStdLog(0xc420060000)
	/home/flisky/.gvm/pkgsets/go1.8/global/src/go.uber.org/zap/global_test.go:105 +0xdd
testing.tRunner(0xc420060000, 0x7baf58)
	/home/flisky/.gvm/gos/go1.8/src/testing/testing.go:657 +0x96
created by testing.(*T).Run
	/home/flisky/.gvm/gos/go1.8/src/testing/testing.go:697 +0x2ca

The old value 4 goes to

go.uber.org/zap.TestNewStdLog(0xc420060000)
	/home/flisky/.gvm/pkgsets/go1.8/global/src/go.uber.org/zap/global_test.go:105 +0xdd

And the new one 2 goes to

go.uber.org/zap.TestNewStdLog.func1(0xc42038eae0, 0xc42010a000)
	/home/flisky/.gvm/pkgsets/go1.8/global/src/go.uber.org/zap/global_test.go:98 +0xed