tools: bpf: handle NULL return in bpf_prog_load_xattr()

bpf_object__open() can return error pointer as well as NULL.
Fix error handling in bpf_prog_load_xattr() (and indirectly
bpf_prog_load()).

Fixes: 6f6d33f3

 ("bpf: selftests add sockmap tests")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>