papi: make socket disconnect more robust

Previusly, read error could happen before message_thread starts,
so do not attempt to join in that case.

Change-Id: Ie119916831e320e38eb02e48515b4c6a9cb937e5
Signed-off-by: Vratko Polak <vrpolak@cisco.com>

src/vpp-api/python/vpp_papi/vpp_transport_socket.py

@@ -149,7 +149,7 @@ class VppTransport(object):
             self.socket.close()
         if self.sque is not None:
             self.sque.put(True)  # Terminate listening thread
-        if self.message_thread is not None:
+        if self.message_thread is not None and self.message_thread.is_alive():
             # Allow additional connect() calls.
             self.message_thread.join()
         # Collect garbage.