iwlwifi: pcie: fix unused txq NULL pointer dereference

Before TVQM, all TX queues were allocated straight at init.
With TVQM, queues are allocated on demand and hence we need
to check if a queue exists before dereferencing it.

Fixes: 66128fa08806 ("iwlwifi: move to TVQM mode")
Signed-off-by: Mordechai Goodstein <mordechay.goodstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>