wifi: iwlwifi: pcie: avoid a NULL pointer dereference

wifi: iwlwifi: pcie: avoid a NULL pointer dereference

It possible that while the rx rb is being handled, the transport has
been stopped and re-started. In this case the tx queue pointer is not
yet initialized, which will lead to a NULL pointer dereference.
Fix it.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20231207044813.cd0898cafd89.I0b84daae753ba9612092bf383f5c6f761446e964@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>