perf session: Fix crash with invalid CPU list

commit 5d67be9 added the option to specify a range of CPUs of interest,
but does not catch an invalid CPU list:

$ perf script -c foo
Segmentation fault (core dumped)

Cc: Anton Blanchard <anton@samba.org>
Link: http://lkml.kernel.org/r/1321206327-5881-1-git-send-email-dsahern@gmail.com
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/session.c

@@ -1333,6 +1333,10 @@ int perf_session__cpu_bitmap(struct perf_session *session,
 	}
 
 	map = cpu_map__new(cpu_list);
+	if (map == NULL) {
+		pr_err("Invalid cpu_list\n");
+		return -1;
+	}
 
 	for (i = 0; i < map->nr; i++) {
 		int cpu = map->map[i];