perf metrics: Avoid segv if default metricgroup isn't set

A metric is default by having "Default" within its groups. The default
metricgroup name needn't be set and this can result in segv in
default_metricgroup_cmp and perf_stat__print_shadow_stats_metricgroup
that assume it has a value when there is a Default metric group. To
avoid the segv initialize the value to "".

Fixes: 1c0e479 ("perf metrics: Sort the Default metricgroup")
Signed-off-by: Ian Rogers <[email protected]>
Reviewed-and-tested-by: Ilkka Koskinen <[email protected]>
Cc: James Clark <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Leo Yan <[email protected]>
Cc: Mike Leach <[email protected]>
Cc: Kajol Jain <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: John Garry <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Namhyung Kim <[email protected]>

tools/perf/util/metricgroup.c

@@ -225,7 +225,7 @@ static struct metric *metric__new(const struct pmu_metric *pm,
 
 	m->pmu = pm->pmu ?: "cpu";
 	m->metric_name = pm->metric_name;
-	m->default_metricgroup_name = pm->default_metricgroup_name;
+	m->default_metricgroup_name = pm->default_metricgroup_name ?: "";
 	m->modifier = NULL;
 	if (modifier) {
 		m->modifier = strdup(modifier);