Try a different approach for lintian-ppa distro_series

cpp/lintian-ppa.cpp

@@ -395,7 +395,9 @@ int main(int argc, char* argv[]) {
     auto ubuntu_opt = lp->distributions["ubuntu"];
     distribution ubuntu = ubuntu_opt.value();
     auto ds_opt = ubuntu.current_series;
-    distro_series current_series = *ds_opt;
+    auto current_series;
+    if (ds_opt)
+        current_series = ds_opt;
 
     // Retrieve user and PPA
     auto user_opt = lp->people[args.user];