smcinvoke: Handle NULL dereference for cb_req

When kernel fails to allocate memory for cb_req,
cb_req remains NULL. Going to out label is not required
since cb_txn till this point is also NULL.

Change-Id: I4f5d5bc9310a726878f306c8b65b131b4ca1e2f1
Signed-off-by: Anmolpreet Kaur <anmolpre@codeaurora.org>