[tls] Ensure that window change is propagated to plainstream interface

The cipherstream xfer_window_changed() message is used to retrigger
the TLS transmit state machine.  If the transmit state machine is
idle, then the window change message will not be propagated to the
plainstream interface.  This can potentially cause the plainstream
interface peer (e.g. httpcore) to block waiting for a window change
message that will never arrive.

Fix by ensuring that the window change message is propagated to the
plainstream interface if the transmit state machine is idle.  (If the
transmit state machine is not idle then the plainstream window will be
zero anyway.)

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tls.c

 		tls->tx_pending &= ~TLS_TX_FINISHED;
 	}
 
-	/* Reschedule process if pending transmissions remain */
-	if ( tls->tx_pending )
+	/* Reschedule process if pending transmissions remain,
+	 * otherwise send notification of a window change.
+	 */
+	if ( tls->tx_pending ) {
 		tls_tx_resume ( tls );
+	} else {
+		xfer_window_changed ( &tls->plainstream );
+	}
 
 	return;