Allocate exact memory size for string buffer to avoid buffer overflow.

Allocate exact memory size for string buffer to avoid buffer overflow.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Ni Ruiyu <ruiyu.ni@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15863 6f19259b-4bc3-4df7-8a09-765794883524