Fix a bug in pipe_t::flush().

Static analysis says:
src\pipe.cpp(193): error V547: Expression is always false. Probably the '||' operator should be used here.

If flush() is called on a pipe whose state was
"terminated" or "double_terminated", the programmer's
intent was to return immediately. But in fact the
two conditions can never be true simultaneously, so
the early return never happens, and we may try to flush
a terminated pipe anyway.

src/pipe.cpp

@@ -190,7 +190,7 @@ void zmq::pipe_t::rollback ()
 void zmq::pipe_t::flush ()
 {
     //  If terminate() was already called do nothing.
-    if (state == terminated && state == double_terminated)
+    if (state == terminated || state == double_terminated)
         return;
 
     //  The peer does not exist anymore at this point.