The http2 code for connection checking needs a transfer to use. Make
sure a working one is set before handler->connection_check() is called.
Reported-by: jnbr on github
Fixes #3541
Closes #3547
/* The protocol has a special method for checking the state of the
connection. Use it to check if the connection is dead. */
unsigned int state;
+ conn->data = data; /* use this transfer for now */
state = conn->handler->connection_check(conn, CONNCHECK_ISDEAD);
dead = (state & CONNRESULT_DEAD);
}