perf trace: Removed duplicated NULL test

No need to test trace.evlist against NULL twice.

Signed-off-by: He Kuang <hekuang@huawei.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1431347316-30401-2-git-send-email-hekuang@huawei.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>