MdeModulePkg/SetupBrowser: Clean the BufferValue for string before use

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>