ivshmem: fix possible 'event' NULL pointer dereference (CA)
ivshmem: fix possible 'event' NULL pointer dereference (CA)

Pointer event->event == NULL can be passed into ObDereferenceObject
in EvtDeviceReleaseHardware, so add the check.

Signed-off-by: Viktor Prutyanov <5d18a6677f95b542ec3825271114e4de0d4487e1@phystech.edu>

YanVugenfirer/kvm-guest-drivers-windows,YanVugenfirer/kvm-guest-drivers-windows,YanVugenfirer/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,YanVugenfirer/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,YanVugenfirer/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows