perf test: check if cpu_map__new() return NULL

It looks like we should check if cpus is NULL after

	cpus = cpu_map__new(NULL);

in test__open_syscall_event_on_all_cpus().

LKML-Reference: <20110114230050.GA7011@localhost>
Signed-off-by: Han Pingtian <phan@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>