ShellPkg: Handle pool allocation failure

FreePool() will receive NULL if AllocateZeroPool() fails.
So a check for NULL is needed.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Sergei Antonov <saproj@gmail.com>
reviewed-by: jaben carsey <jaben.carsey@gmail.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/ShellPkg@14689 6f19259b-4bc3-4df7-8a09-765794883524