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: 32e93fb7f6 ("{f,m}_bpf: allow for sharing maps")
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Signed-off-by: David Ahern <dsahern@kernel.org>

lib/bpf_legacy.c

@@ -2992,7 +2992,7 @@ static int bpf_obj_open(const char *pathname, enum bpf_prog_type type,
 out:
 	bpf_elf_ctx_destroy(ctx, ret < 0);
 	if (ret < 0) {
-		if (fd)
+		if (fd >= 0)
 			close(fd);
 		return ret;
 	}