buffer.c: check whether buf is NULL in fuse_bufvec_advance func

In fuse_bufvec_advance func, calling fuse_bufvec_current func
may return NULL, so we should check whether buf is NULL before
using it.

Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
Signed-off-by: Haotian Liu <lihaotian9@huawei.com>

lib/buffer.c

@@ -266,6 +266,9 @@ static int fuse_bufvec_advance(struct fuse_bufvec *bufv, size_t len)
 {
 	const struct fuse_buf *buf = fuse_bufvec_current(bufv);
 
+	if (!buf)
+		return 1;
+
 	bufv->off += len;
 	assert(bufv->off <= buf->size);
 	if (bufv->off == buf->size) {