PCI: Always have realloc_head in __assign_resources_sorted()

Add a dummy list to always have a non-NULL realloc head in
__assign_resources_sorted() as it allows only checking list_empty().

In future, it would be good to ensure all callers provide a valid
realloc_head but that is relatively complex to do in practice and not
necessary for the subsequent optional resource handling fix.

Link: https://lore.kernel.org/r/20241216175632.4175-21-ilpo.jarvinen@linux.intel.com


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Xiaochun Lee <lixc17@lenovo.com>