pci-assign: Fix a bug when map MSI-X table memory failed

When mmapping memory for the MSI-X table failsthe dev->msix_table is
not set to NULL and assigned_dev_unregister_msix_mmio() will cause
a segfault when trying to munmap it.
Signed-off-by: NGonglei Arei <arei.gonglei@huawei.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NGonglei <arei.gonglei@huawei.com>

hw/i386/kvm/pci-assign.c

@@ -1664,6 +1664,7 @@ static void assigned_dev_register_msix_mmio(AssignedDevice *dev, Error **errp)
                            MAP_ANONYMOUS|MAP_PRIVATE, 0, 0);
     if (dev->msix_table == MAP_FAILED) {
         error_setg_errno(errp, errno, "failed to allocate msix_table");
+        dev->msix_table = NULL;
         return;
     }