rgw: fix crash when listing objects via swift

Fixes: http://tracker.ceph.com/issues/19249

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
(cherry picked from commit a9ec5e8)

Conflicts:
        src/rgw/rgw_rest_swift.cc (kraken does not have
           5cf5ab4, so retain kraken
           rgw_obj_key& type)

src/rgw/rgw_rest_swift.cc

@@ -282,15 +282,18 @@ void RGWListBucket_ObjStore_SWIFT::send_response()
   while (iter != objs.end() || pref_iter != common_prefixes.end()) {
     bool do_pref = false;
     bool do_objs = false;
-    rgw_obj_key& key = iter->key;
+    rgw_obj_key& key;
+    if (iter != objs.end()) {
+      key = iter->key;
+    }
     if (pref_iter == common_prefixes.end())
       do_objs = true;
     else if (iter == objs.end())
       do_pref = true;
-    else if (key.name.compare(pref_iter->first) == 0) {
+    else if (!key.empty() && key.name.compare(pref_iter->first) == 0) {
       do_objs = true;
       ++pref_iter;
-    } else if (key.name.compare(pref_iter->first) <= 0)
+    } else if (!key.empty() && key.name.compare(pref_iter->first) <= 0)
       do_objs = true;
     else
       do_pref = true;