Avoid poping queue if empty

lib/include/srsran/common/tti_sempahore.h

@@ -75,8 +75,10 @@ public:
   {
     std::unique_lock<std::mutex> lock(mutex);
 
-    // Pop first element
+    // If the FIFO is not empty pop first element
+    if (not fifo.empty()) {
       fifo.pop_front();
+    }
 
     // Notify release
     cvar.notify_all();