Symfony version(s) affected: 3.4 up till current master

Description
When making a request, the TraceableEventDispatcher opens a section with the Stopwatch, which never gets closed.

This causes a \LogicException when measuring the time of something with an internal requests.

How to reproduce

$stopwatch->start('foo');
// do something with an internal request:
$kernel->handle($subRequest, HttpKernelInterface::SUB_REQUEST, false);
$stopwatch->stop('foo');

--> yields \LogicException('Event "foo" is not started.') in the last line

Possible Solution
Call stopSection even if no 'X-Debug-Token' is set in the response headers.

The route name might be a neat fallback:

$token = $event->getResponse()->headers->get('X-Debug-Token', $event->getRequest()->attributes->get('_route'));

The TraceableEventDispatcher starts the section, it should be responsible for it being closed again, no matter what.

Additional context
This happens running a functional test with phpunit with debug enabled.

The only place where 'X-Debug-Token' is set seems to be the Profiler - which is probably not enabled during unit tests (at least for us).