When copy new string content to BufferValue, need to clean the BufferValue firstly, or the BufferValue may contain some content that doesn't belong to the new string. Cc: Liming Gao <liming.gao@intel.com> Cc: Eric Dong <eric.dong@intel.com> Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Dandan Bi <dandan.bi@intel.com> Reviewed-by: Eric Dong <eric.dong@intel.com> Reviewed-by: Liming Gao <liming.gao@intel.com>