fix block_queue pop and clear methods

- pop was bzero'ing the popped object which
  causes for more complex objects like std::string
- clear wasn't actually emptying the queue

lib/include/srslte/common/block_queue.h

@@ -109,7 +109,6 @@ public:
 
   myobj wait_pop() { // blocking pop
     myobj value;
-    bzero(&value, sizeof(myobj));
     pop_(&value, true);
     return value;
   }
@@ -154,8 +153,8 @@ private:
     }
     if (value) {
       *value = q.front();
-      q.pop();
     }
+    q.pop();
     ret = true;
     if (mutexed_callback) {
       mutexed_callback->popping(*value);