MdeModulePkg/PciBusDxe: Fix VS2012 build failure

MdeModulePkg/PciBusDxe: Fix VS2012 build failure

Initialize local variable to suppress warning C4703:
potentially uninitialized local pointer variable.

Both reads (dereferences) of "PciRootBridgeIo" in
PciBusDriverBindingStart() are only reached if
"gFullEnumeration" is TRUE on entry *and* we successfully
open the EfiPciRootBridgeIoProtocol interface.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>