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>

xen/drivers/vpci/vpci.c

@@ -37,7 +37,7 @@ extern vpci_register_init_t *const __end_vpci_array[];
 
 void vpci_remove_device(struct pci_dev *pdev)
 {
-    if ( !has_vpci(pdev->domain) )
+    if ( !has_vpci(pdev->domain) || !pdev->vpci )
         return;
 
     spin_lock(&pdev->vpci->lock);
@@ -326,7 +326,7 @@ uint32_t vpci_read(pci_sbdf_t sbdf, unsigned int reg, unsigned int size)
 
     /* Find the PCI dev matching the address. */
     pdev = pci_get_pdev(d, sbdf);
-    if ( !pdev )
+    if ( !pdev || !pdev->vpci )
         return vpci_read_hw(sbdf, reg, size);
 
     spin_lock(&pdev->vpci->lock);
@@ -436,7 +436,7 @@ void vpci_write(pci_sbdf_t sbdf, unsigned int reg, unsigned int size,
      * Passthrough everything that's not trapped.
      */
     pdev = pci_get_pdev(d, sbdf);
-    if ( !pdev )
+    if ( !pdev || !pdev->vpci )
     {
         vpci_write_hw(sbdf, reg, size, data);
         return;