ivshmem: check the value returned by fstat()

The function fstat() may fail, so check its return value.

Acked-by: Levente Kurusa <lkurusa@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
