perf evsel: Fix potential null pointer dereference in perf_evsel__new_idx()

If evsel is NULL, we should return NULL to avoid a NULL pointer
dereference a bit later in the code.

Signed-off-by: Hisao Tanabe <xtanabe@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 03e0a7df ("perf tools: Introduce bpf-output event")
LPU-Reference: 20180824154556.23428-1-xtanabe@gmail.com
Link: https://lkml.kernel.org/n/tip-e5plzjhx6595a5yjaf22jss3@git.kernel.org


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