IntelFrameworkModulePkg BootMngr: Fix potential read over memory boundary

IntelFrameworkModulePkg BootMngr: Fix potential read over memory boundary

This commit will resolve the issue brought by r17737.

HelpString = AllocateCopyPool (HelpSize, L"Device Path : ");

The above using of AllocateCopyPool() will read contents out of the scope
of the constant string. Potential risk for the constant string allocated
at the boundary of memory region.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17932 6f19259b-4bc3-4df7-8a09-765794883524