lib: bpf_legacy: avoid to pass invalid argument to close()

lib: bpf_legacy: avoid to pass invalid argument to close()

In function bpf_obj_open, if bpf_fetch_prog_arg() return an error, we
end up in the out: path with a negative value for fd, and pass it to
close.

Avoid this checking for fd to be positive.

Fixes: 32e93fb7f66d ("{f,m}_bpf: allow for sharing maps")
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Signed-off-by: David Ahern <dsahern@kernel.org>