Avoid segfault for anonymous clients with macaroons (#39)

Don't dereference a null sentryPtr

Author: jthiltges

src/UserSentry.hh

@@ -76,8 +76,8 @@ public:
         std::string username;
         auto got_token = client->eaAPI->Get("request.name", username);
         if (!got_token && (!client->name || !client->name[0])) {
-            log.Emsg("UserSentry", "Anonymous client; no user set, cannot change FS UIDs");
-            m_is_anonymous = true;
+            // Anonymous client; no user set
+            this->Init("", log);
             return;
         }
 
@@ -141,6 +141,12 @@ public:
 
         int retval;
 
+        if (username.empty()) {
+            log.Emsg("UserSentry", "Anonymous client; no user set, cannot change FS UIDs");
+            m_is_anonymous = true;
+            return;
+        }
+
         do {
             retval = getpwnam_r(username.c_str(), &pwd, &buf[0], buflen, &result);
             if ((result == nullptr) && (retval == ERANGE)) {

src/multiuser.cpp

@@ -246,7 +246,8 @@ class MultiuserChecksum : public XrdCksWrapper
                 if (username) {
                     return new UserSentry(username, *m_log);
                 } else {
-                    return nullptr;
+                    // Anonymous requests will not have a request.name
+                    return new UserSentry("", *m_log);
                 }
             }
         }