sserver: Fix a potential memory exception

src/StratumServer.cc

@@ -318,11 +318,11 @@ void JobRepository::tryCleanExpiredJobs() {
       break; // not expired
     }
 
-    // remove expired job
-    exJobs_.erase(itr);
-
     LOG(INFO) << "remove expired stratum job, id: " << itr->first
               << ", time: " << date("%F %T", jobTime);
+
+    // remove expired job
+    exJobs_.erase(itr);
   }
 }