ShellPkg: Code refine. Add error handling code to check pointer and remove redundant 'ASSERT'.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Jaben Carsey <Jaben.carsey@intel.com>


git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/ShellPkg@16525 6f19259b-4bc3-4df7-8a09-765794883524