perf metrics: Avoid segv with --topdown for metrics without a group

Some metrics may not have a metric_group which can result in segvs
with "perf stat --topdown". Add a condition for the no metric_group
case.

Fixes: 1647cd5 ("perf stat: Implement --topdown using json metrics")
Reported-by: Kan Liang <[email protected]>
Signed-off-by: Ian Rogers <[email protected]>
Tested-by: Kan Liang <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kajol Jain <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: captain5050

tools/perf/util/metricgroup.c

@@ -1672,7 +1672,7 @@ static int metricgroup__topdown_max_level_callback(const struct pmu_metric *pm,
 {
 	unsigned int *max_level = data;
 	unsigned int level;
-	const char *p = strstr(pm->metric_group, "TopdownL");
+	const char *p = strstr(pm->metric_group ?: "", "TopdownL");
 
 	if (!p || p[8] == '\0')
 		return 0;