perf tests: Check return of perf_evlist__open sw clock event period test

We were not checking if we successfully opened the counters, i.e. if
sys_perf_event_open worked, when it doesn't in this test, we were
continuing anyway and then segfaulting when trying to access the file
descriptor array, that at that point had been freed in perf_evlist__open
error path:

[root@ssdandy ~]# perf test -v 19
19: Test software clock events have valid period values    :
--- start ---
Segmentation fault (core dumped)
[root@ssdandy ~]#

Do the check and bail out instead.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-6qy8ljkn0e9hm7bh7keo5z68@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/sw-clock.c

@@ -57,7 +57,11 @@ static int __test__sw_clock_freq(enum perf_sw_ids clock_id)
 		goto out_delete_maps;
 	}
 
-	perf_evlist__open(evlist);
+	if (perf_evlist__open(evlist)) {
+		err = -errno;
+		pr_debug("Couldn't open evlist: %s\n", strerror(errno));
+		goto out_delete_maps;
+	}
 
 	err = perf_evlist__mmap(evlist, 128, true);
 	if (err < 0) {