iwlwifi: pcie: don't use vid 0

In cases of hardware or DMA error, the vid read from
a zeroed location will be 0, and we will access the rxb
at index 0 in the global table, while it may be NULL or
owned by hardware.
Invalidate vid 0 in order to detect the situation and
bail out.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>