libperf: Add NULL pointer check for cpu_map iteration and NULL assignment for all_cpus.

A NULL pointer may be passed to perf_cpu_map__cpu and then cause a
crash, such as the one commit cb71f7d4 ("libperf: Setup initial
evlist::all_cpus value") fix.
Signed-off-by: He Zhe <zhe.he@windriver.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Kyle Meyer <meyerk@hpe.com>
Link: http://lore.kernel.org/lkml/1583665157-349023-1-git-send-email-zhe.he@windriver.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/perf/cpumap.c

@@ -247,7 +247,7 @@ struct perf_cpu_map *perf_cpu_map__new(const char *cpu_list)
 
 int perf_cpu_map__cpu(const struct perf_cpu_map *cpus, int idx)
 {
-	if (idx < cpus->nr)
+	if (cpus && idx < cpus->nr)
 		return cpus->map[idx];
 
 	return -1;

tools/lib/perf/evlist.c

@@ -125,6 +125,7 @@ void perf_evlist__exit(struct perf_evlist *evlist)
 	perf_cpu_map__put(evlist->cpus);
 	perf_thread_map__put(evlist->threads);
 	evlist->cpus = NULL;
+	evlist->all_cpus = NULL;
 	evlist->threads = NULL;
 	fdarray__exit(&evlist->pollfd);
 }