Check invariants earlier in vconn and stream code.

These invariants are checked by vconn_open() and stream_open(), but there
is no reason not to check them earlier also.  vconn and stream creation
don't have to go through vconn_open() and stream_open(), so this ensures
that the invariants get checked either way.

lib/stream.c

@@ -476,6 +476,7 @@ stream_init(struct stream *stream, struct stream_class *class,
                     : SCS_DISCONNECTED);
     stream->error = connect_status;
     stream->name = xstrdup(name);
+    assert(stream->state != SCS_CONNECTING || class->connect);
 }
 
 void