NetworkPkg/SnpDxe: Prevent invalid PCI BAR access
    
    REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1563
    
    SnpDxe initializes values for MemoryBarIndex and IoBarIndex to 0 and 1
    respectively even if calls to PciIo->GetBarAttributes never return
    success.
    
    Later, if the BAR is used to perform IO/Mem reads/writes, a potentially
    non-existent BAR index may be accessed. This change initializes the
    values
    to an invalid BAR index (PCI_MAX_BAR) so the condition can be explicitly
    checked to avoid an invalid BAR access.
    
    Cc: Siyuan Fu <siyuan.fu@intel.com>
    Cc: Maciej Rabeda <maciej.rabeda@linux.intel.com>
    Cc: Jiaxin Wu <jiaxin.wu@intel.com>
    Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
    Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>
    Reviewed-by: Maciej Rabeda <maciej.rabeda@linux.intel.com>