benchstat: omit 0 values from geomean
benchstat: omit 0 values from geomean

This makes the geomean undefined (or zero, depending on who you ask).
Since these usually come from counts, it's more informative to just
omit the benchmark.

Change-Id: Ia426955784cf12fea19b3d5e51a49a436cbc81c6
Reviewed-on: https://go-review.googlesource.com/38092
Reviewed-by: Quentin Smith <d6b8e48afb2534b213e391cab43016505747a234@golang.org>

golang/perf,golang/perf