Fix possible out-of-bounds access detected by Ulrich Spörleins "scan-build".
Some invalid PCI device selectors could cause read access to an initialized
variable next to the array (local loop index variable).

While here, the parser has been made more strict with regard to the syntax
of PCI device selectors as documented in the man-page. E.g. "pci:" used to
be interpreted as "pci0:0".

MFC after: 3 days