tools lib bpf: Make bpf_program__get_private() use IS_ERR()

For consistency with bpf_map__priv() and elsewhere.

Acked-by: Wang Nan <wangnan0@huawei.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/n/tip-x17nk5mrazkf45z0l0ahlmo8@git.kernel.org


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