perf stats: Fix divide by 0 in variance

Number of samples needs to be greater 1 to have a variance.

Fixes nan% in perf-kvm-live output.
Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Runzhen Wang <runzhen@linux.vnet.ibm.com>
Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1369527896-3650-9-git-send-email-dsahern@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/stat.c

@@ -37,7 +37,7 @@ double stddev_stats(struct stats *stats)
 {
 	double variance, variance_mean;
 
-	if (!stats->n)
+	if (stats->n < 2)
 		return 0.0;
 
 	variance = stats->M2 / (stats->n - 1);