PCI: Check parent for NULL in of_pci_bus_release_domain_nr()

PCI: Check parent for NULL in of_pci_bus_release_domain_nr()

of_pci_bus_find_domain_nr() allows its parent parameter to be NULL but
of_pci_bus_release_domain_nr() (that undoes its effect) doesn't -- that
means it's going to blow up while calling of_get_pci_domain_nr() if the
parent parameter indeed happens to be NULL.  Add the missing NULL check.

Found by Linux Verification Center (linuxtesting.org) with the Svace static
analysis tool.

Fixes: c14f7ccc9f5d ("PCI: Assign PCI domain IDs by ida_alloc()")
Signed-off-by: Sergey Shtylyov <s.shtylyov@auroraos.dev>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260127203944.28588-1-s.shtylyov@auroraos.dev