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>
