back() and front() have defined behaviour now when queue is empty

Adapter.cc

@@ -147,7 +147,13 @@ namespace pEp {
 
         bool in_shutdown()
         {
-            SYNC_EVENT ev = q.back();
+            SYNC_EVENT ev;
+            try {
+                ev = q.back();
+            }
+            catch (std::underflow_error&) {
+                return false;
+            }
             if (ev)
                 return false;
             else

locked_queue.hh

@@ -38,17 +38,21 @@ namespace utility
             _q.clear();
         }
 
-        // undefined behavior when queue empty
+        // defined behavior when queue empty
         T back()
         {
             Lock lg(_mtx);
+            if (_q.empty())
+                throw std::underflow_error("queue empty");
             return _q.back();
         }
 
-        // undefined behavior when queue empty
+        // defined behavior when queue empty
         T front()
         {
             Lock lg(_mtx);
+            if (_q.empty())
+                throw std::underflow_error("queue empty");
             return _q.front();
         }