perf evlist: Avoid iteration for empty evlist.

As seen with 'perf stat --null ..' and reported in:
https://lore.kernel.org/lkml/YjCLcpcX2peeQVCH@kernel.org/

v2. Avoids setting evsel in the empty list case as suggested by Jiri Olsa.

    Committer testing:

Before:

  $  perf stat --null sleep 1
  Segmentation fault (core dumped)
  $

After:

  $  perf stat --null sleep 1

   Performance counter stats for 'sleep 1':

         1.010340646 seconds time elapsed

         0.001420000 seconds user
         0.000000000 seconds sys
  $

Fixes: 472832d2 ("perf evlist: Refactor evlist__for_each_cpu()")
Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20220317231643.550902-1-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evlist.c

@@ -346,7 +346,7 @@ struct evlist_cpu_iterator evlist__cpu_begin(struct evlist *evlist, struct affin
 {
 	struct evlist_cpu_iterator itr = {
 		.container = evlist,
-		.evsel = evlist__first(evlist),
+		.evsel = NULL,
 		.cpu_map_idx = 0,
 		.evlist_cpu_map_idx = 0,
 		.evlist_cpu_map_nr = perf_cpu_map__nr(evlist->core.all_cpus),
@@ -354,17 +354,23 @@ struct evlist_cpu_iterator evlist__cpu_begin(struct evlist *evlist, struct affin
 		.affinity = affinity,
 	};
 
+	if (evlist__empty(evlist)) {
+		/* Ensure the empty list doesn't iterate. */
+		itr.evlist_cpu_map_idx = itr.evlist_cpu_map_nr;
+	} else {
+		itr.evsel = evlist__first(evlist);
 		if (itr.affinity) {
 			itr.cpu = perf_cpu_map__cpu(evlist->core.all_cpus, 0);
 			affinity__set(itr.affinity, itr.cpu.cpu);
 			itr.cpu_map_idx = perf_cpu_map__idx(itr.evsel->core.cpus, itr.cpu);
 			/*
-		 * If this CPU isn't in the evsel's cpu map then advance through
-		 * the list.
+			 * If this CPU isn't in the evsel's cpu map then advance
+			 * through the list.
 			 */
 			if (itr.cpu_map_idx == -1)
 				evlist_cpu_iterator__next(&itr);
 		}
+	}
 	return itr;
 }