iwlwifi: fix freeing uninitialized pointer

If on iwl_dump_nic_event_log() error occurs before that function
initialize buf, we process uninitiated pointer in
iwl_dbgfs_log_event_read() and can hit "BUG at mm/slub.c:3409"

Resolves:
https://bugzilla.redhat.com/show_bug.cgi?id=951241



Cc: stable@vger.kernel.org
Reported-by:  <ian.odette@eprize.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>