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

tools/virtiofsd/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: Haotian Li <lihaotian9@huawei.com>
Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
Message-Id: <29fc87c2-b87c-4c34-40d4-75381f228849@huawei.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>