NetworkPkg: Update PXE driver to check for NULL pointer before use it.

NetworkPkg: Update PXE driver to check for NULL pointer before use it.

This patch is to fix the issue that dereferencing of "This" (EFI_LOAD_FILE_PROTOCOL)
in EfiPxeLoadFile() is happening before the NULL check.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Sriram Subramanian <sriram-s@hpe.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>