s390x/pci: check for invalid function handle

broken guest may provide 0 (invalid) function handle to zpci
instructions. Since we use function handle 0 to indicate an empty
slot in the PHB we have to add an additional check to spot this
kind of error.

Signed-off-by: Frank Blaschka <blaschka@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>

hw/s390x/s390-pci-bus.c

@@ -170,7 +170,7 @@ S390PCIBusDevice *s390_pci_find_dev_by_fh(uint32_t fh)
     S390pciState *s = S390_PCI_HOST_BRIDGE(
         object_resolve_path(TYPE_S390_PCI_HOST_BRIDGE, NULL));
 
-    if (!s) {
+    if (!s || !fh) {
         return NULL;
     }