Fix use-after-free in DoH write buffer after HTTP/2 send

After the send callback completes, the UV request is freed but
the HTTP/2 socket's write buffer still points to the freed memory.
If nghttp2 subsequently needs to send frames (e.g. SETTINGS ACK),
the server_read_callback reads from the dangling buffer.

Clear the write buffer before freeing the UV request.

lib/isc/netmgr/http.c

@@ -2743,6 +2743,8 @@ server_httpsend(isc_nmhandle_t *handle, isc_nmsocket_t *sock,
 	} else {
 		cb(handle, result, cbarg);
 	}
+
+	isc_buffer_initnull(&sock->h2->wbuf);
 	isc__nm_uvreq_put(&req);
 }