[2.4][HttpKernel] Fix profiler event-listener usage outside request stack context

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

The bug has been reported at https://github.com/silexphp/Silex-WebProfiler/issues/27

todo : 
 - [x] add unit test