Assert queue empty before asking anything

hpib.py

@@ -51,6 +51,8 @@ class GPIBProtocol(serial.threaded.LineReader):
              RuntimeError: if no response was received to a query.
         """
         with self.lock:
+            assert(responses.empty())
+
             if address:
                 self.write_line("++addr {:d}".format(address))
             self.write_line(command)