vpci: don't assume that vpci per-device data exists unconditionally

vpci: don't assume that vpci per-device data exists unconditionally

It's possible for a device to be assigned to a domain but have no
vpci structure if vpci_process_pending() failed and called
vpci_remove_device() as a result.  The unconditional accesses done by
vpci_{read,write}() and vpci_remove_device() to pdev->vpci would
then trigger a NULL pointer dereference.

Add checks for pdev->vpci presence in the affected functions.

Fixes: 9c244fdef7 ('vpci: add header handlers')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>