MdeModulePkg/SetupBrowser: Clean the BufferValue for string before use
    
    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>