MdeModulePkg/UdfDxe: Remove negative comparison of unsigned number
    
    This patch gets rid of a negative comparison of an UINT64 type (Offset)
    as it'll never evaluate to true.
    
    Cc: Star Zeng <star.zeng@intel.com>
    Cc: Eric Dong <eric.dong@intel.com>
    Cc: Ruiyu Ni <ruiyu.ni@intel.com>
    Cc: Dandan Bi <dandan.bi@intel.com>
    Contributed-under: TianoCore Contribution Agreement 1.1
    Reported-by: Star Zeng <star.zeng@intel.com>
    Signed-off-by: Paulo Alcantara <pcacjr@zytor.com>
    Reviewed-by: Star Zeng <star.zeng@intel.com>