Make sure "BootFileName" is not overloaded before use it in PXE driver. Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: chenzhihui <chenzhihui4@huawei.com> Signed-off-by: Heyi Guo <heyi.guo@linaro.org> Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>