PCI: Allow release of resources that were never assigned

It is entirely possible that the BIOS wasn't able to assign resources to a
device. In this case don't crash in pci_release_resource() when we try to
resize the resource.

Fixes: 8bb705e ("PCI: Add pci_resize_resource() for resizing BARs")
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
CC: stable@vger.kernel.org	# v4.15+

drivers/pci/setup-res.c

@@ -401,6 +401,10 @@ void pci_release_resource(struct pci_dev *dev, int resno)
 	struct resource *res = dev->resource + resno;
 
 	pci_info(dev, "BAR %d: releasing %pR\n", resno, res);
+
+	if (!res->parent)
+		return;
+
 	release_resource(res);
 	res->end = resource_size(res) - 1;
 	res->start = 0;