ShellPkg: Fix memory leak when running Shell script.

ShellPkg: Fix memory leak when running Shell script.

When we run following script in Shell:
"
for %a run (1 200)
  echo %a
  memmap
endfor
"
We may find memory leak in system. This patch free buffer in 'BufferToFreeList' to avoid this issue.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19521 6f19259b-4bc3-4df7-8a09-765794883524