iwlwifi: Fix null pointer referencing in iwl_dbgfs_rx_queue_read.

Test for null pointer prior to access.
Print "Not Allocated" if null pointer.

Signed-off-by: Dor Shaish <dor.shaish@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>