Problem: test_security_zap occasionally segfaults

Solution: check if a session's _pipe has been allocated before using
it, since as a consequence of creating the pipes after the handshake
it's no longer guaranteed to be there.

(cherry picked from commit 350b4b34f460b91b8fa8f692cf6bc30d561a5711)

Conflicts:
	src/session_base.cpp

src/session_base.cpp

@@ -239,7 +239,8 @@ void zmq::session_base_t::read_activated (pipe_t *pipe_)
     }
 
     if (unlikely (engine == NULL)) {
-        pipe->check_read ();
+        if (pipe)
+            pipe->check_read ();
         return;
     }