[libc++] Fix bugs in geomean calculation

Author: ldionne

libcxx/utils/compare-benchmarks

@@ -69,10 +69,10 @@ def plain_text_comparison(data, metric, baseline_name=None, candidate_name=None)
     table = data[['benchmark', f'{metric}_0', f'{metric}_1', 'difference', 'percent']]
 
     # Compute the geomean and report on their difference
-    geomean_0 = statistics.geometric_mean(data[f'{metric}_0'])
-    geomean_1 = statistics.geometric_mean(data[f'{metric}_1'])
+    geomean_0 = statistics.geometric_mean(data[f'{metric}_0'].dropna())
+    geomean_1 = statistics.geometric_mean(data[f'{metric}_1'].dropna())
     geomean_row = ['Geomean', geomean_0, geomean_1, (geomean_1 - geomean_0), (geomean_1 - geomean_0) / geomean_0]
-    table.loc[len(table)] = geomean_row
+    table.loc[table.index.max()] = geomean_row
 
     return tabulate.tabulate(table.set_index('benchmark'), headers=headers, floatfmt=fmt, numalign='right')