perf bpf: Fix NULL return handling in bpf__prepare_load()

bpf_object__open()/bpf_object__open_buffer can return error pointer or
NULL, check the return values with IS_ERR_OR_NULL() in bpf__prepare_load
and bpf__prepare_load_buffer

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: netdev@vger.kernel.org
Link: https://lkml.kernel.org/n/tip-psf4xwc09n62al2cb9s33v9h@git.kernel.org


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