The pointer argument should be set to NULL if not used not FALSE.

The pointer argument should be set to NULL if not used not FALSE.
  FALSE evaluates to 0. This was flagged by LLVM compiler as a
  warning:
  "expression which evaluates to zero treated as a null pointer
   constant of type 'EFI_MTFTP4_OVERRIDE_DATA *'
   [-Wnon-literal-null-conversion]"

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Harry Liebel <Harry.Liebel@arm.com>
Reviewed-By: Olivier Martin <olivier.martin@arm.com>
Reviewed-By: Fu, Siyuan <siyuan.fu@intel.com>
Reviewed-By: Ye, Ting (ting.ye@intel.com)
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15741 6f19259b-4bc3-4df7-8a09-765794883524