pci_iomap() treats pci_resource_start()'s -1 failure sentinel as a valid base and drives __ioremap_common() into BUG_ON panic for any absent/unallocatable BAR

Summary

The shim guards only base==0 but the getter signals failure with -1 - a missing/unallocatable BAR feeds ~0 into ioremap where BUG_ON(phys_addr & PAGE_MASK) is guaranteed true. Attach-time panic on quirky/malicious PCIe hardware; every in-tree caller is written to the NULL contract. Validate-both-sentinels fix.