MdeModulePkg: Clear the buffer after using it

Cc: Liming Gao <liming.gao@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdeModulePkg/Universal/DisplayEngineDxe/ProcessOptions.c

@@ -907,7 +907,8 @@ PasswordProcess (
 
     Status = EFI_INVALID_PARAMETER;
   }
-  
+  ZeroMem (TempString, (Maximum + 1) * sizeof (CHAR16));
+  ZeroMem (StringPtr, (Maximum + 1) * sizeof (CHAR16));
   FreePool (TempString);
   FreePool (StringPtr);