cli: Filter out zero values prior to calculating harmonic mean

The stressng-sync-file-64 resultclass from the
mmtests/stressng-class-io-parallel benchmark has been found to
occasionally return 0.0 values. But the resultclass specifies harmonic
mean, which effectively means we end up dividing by zero and getting
weird results. I anticipate that there is a bug in the benchmark
somewhere.

To work around this gracefully, let's detect the case, emit a warning
and filter out all the zero values.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>