int git_smart__push(git_transport *transport, git_push *push, const git_remote_callbacks *cbs)
{
	transport_smart *t = (transport_smart *)transport;
	struct push_packbuilder_payload packbuilder_payload = {0};
	git_buf pktline = GIT_BUF_INIT;
	int error = 0, need_pack = 0;
	push_spec *spec;
	unsigned int i;

	packbuilder_payload.pb = push->pb;

	if (cbs && cbs->push_transfer_progress) {
		packbuilder_payload.cb = cbs->push_transfer_progress;
		packbuilder_payload.cb_payload = cbs->payload;
	}

#ifdef PUSH_DEBUG
{
	git_remote_head *head;
	char hex[GIT_OID_HEXSZ+1]; hex[GIT_OID_HEXSZ] = '\0';

	git_vector_foreach(&push->remote->refs, i, head) {
		git_oid_fmt(hex, &head->oid);
		fprintf(stderr, "%s (%s)\n", hex, head->name);
	}

	git_vector_foreach(&push->specs, i, spec) {
		git_oid_fmt(hex, &spec->roid);
		fprintf(stderr, "%s (%s) -> ", hex, spec->lref);
		git_oid_fmt(hex, &spec->loid);
		fprintf(stderr, "%s (%s)\n", hex, spec->rref ?
			spec->rref : spec->lref);
	}
}
#endif

	/*
	 * Figure out if we need to send a packfile; which is in all
	 * cases except when we only send delete commands
	 */
	git_vector_foreach(&push->specs, i, spec) {
		if (spec->refspec.src && spec->refspec.src[0] != '\0') {
			need_pack = 1;
			break;
		}
	}

	if ((error = git_smart__get_push_stream(t, &packbuilder_payload.stream)) < 0 ||
		(error = gen_pktline(&pktline, push)) < 0 ||
		(error = packbuilder_payload.stream->write(packbuilder_payload.stream, git_buf_cstr(&pktline), git_buf_len(&pktline))) < 0)
		goto done;

	if (need_pack &&
		(error = git_packbuilder_foreach(push->pb, &stream_thunk, &packbuilder_payload)) < 0)
		goto done;

	/* If we sent nothing or the server doesn't support report-status, then
	 * we consider the pack to have been unpacked successfully */
	if (!push->specs.length || !push->report_status)
		push->unpack_ok = 1;
	else if ((error = parse_report(t, push)) < 0)
		goto done;

	/* If progress is being reported write the final report */
	if (cbs && cbs->push_transfer_progress) {
		error = cbs->push_transfer_progress(
					push->pb->nr_written,
					push->pb->nr_objects,
					packbuilder_payload.last_bytes,
					cbs->payload);

		if (error < 0)
			goto done;
	}

	if (push->status.length) {
		error = update_refs_from_report(&t->refs, &push->specs, &push->status);
		if (error < 0)
			goto done;

		error = git_smart__update_heads(t, NULL);
	}

done:
	git_buf_free(&pktline);
	return error;
}

The product dereferences a pointer that it expects to be valid but is NULL.