iwlwifi: pcie: fix to correct null check

The fixes made in commit: 4ae57980 ("iwlwifi: pcie: add a NULL check in
iwl_pcie_txq_unmap") is not enough in 4.4.y tree.. This still have problems
with null references. This provides the correct fix.
Also, this is a problem only in 4.4.y. This patch has been applied to other
LTS trees, but with the correct fixes.

Fixes: 4ae57980 ("iwlwifi: pcie: add a NULL check in iwl_pcie_txq_unmap")
Cc: stable@vger.kernel.org
Cc: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Cc: Luca Coelho <luciano.coelho@intel.com>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: Sasha Levin <sashal@kernel.org>
Signed-off-by: Nobuhiro Iwamatsu (CIP) <nobuhiro1.iwamatsu@toshiba.co.jp>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>