fix panic if the GOOS doesn't support strict IEEE

[snyh]

the default TimerGranularity is zero, and divide zero will cause SIGFPE if the code doesn't support strict IEEE.

This will be happen in gccgo under some special platform.

  panic: runtime error: floating point error
  [signal SIGFPE: floating-point exception code=7 addr=4859750620 pc=4859750621]

And check the zero will improve the default performances by removing the operations about time.Now.

[hashicorp-cla]

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