alpha/PCI: Fix __pci_mmap_fits() overflow for zero-length BARs

alpha/PCI: Fix __pci_mmap_fits() overflow for zero-length BARs

Currently, __pci_mmap_fits() computes the BAR size using
"pci_resource_len() - 1", which wraps to a large value when the BAR length
is zero, causing the bounds check to incorrectly succeed.

Add an early return for empty resources.

Fixes: 10a0ef39fbd1 ("PCI/alpha: pci sysfs resources")
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Magnus Lindholm <linmag7@gmail.com>
Tested-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Acked-by: Magnus Lindholm <linmag7@gmail.com>
Link: https://patch.msgid.link/20260508043543.217179-15-kwilczynski@kernel.org