iwlwifi: pcie: add a NULL check in iwl_pcie_txq_unmap

I hit a NULL pointer exception in this function when the
init flow went really bad.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20210115130252.2e8da9f2c132.I0234d4b8ddaf70aaa5028a20c863255e05bc1f84@changeid