ShellPkg: Handle pool allocation failure

If AllocateZeroPool() returns NULL,
GetVariable() will return EFI_BUFFER_TOO_SMALL,
FreePool() will receive NULL.
So check for NULL before FreePool().

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Sergei Antonov <saproj@gmail.com>
Reviewed-by: Jaben Carsey <Jaben.carsey@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14688 6f19259b-4bc3-4df7-8a09-765794883524