minor #15483 fix debug-ext 003.phpt (glensc)

This PR was submitted for the 2.8 branch but it was merged into the 2.7 branch instead (closes #15483).

Discussion
----------

fix debug-ext 003.phpt

in my environment the filename is filled, not "-"

$ php --version
PHP 5.5.27 (cli) (built: Jul 17 2015 12:32:05)
Copyright (c) 1997-2015 The PHP Group
Zend Engine v2.5.0, Copyright (c) 1998-2015 Zend Technologies

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
-------

18b6964 fix debug-ext 003.phpt

src/Symfony/Component/Debug/Resources/ext/tests/003.phpt

@@ -56,7 +56,7 @@ Symfony\Component\Debug\Exception\UndefinedFunctionException Object
     [message:protected] => Attempted to call function "notexist" from namespace "Symfony\Component\Debug".
     [string:Exception:private] => 
     [code:protected] => 0
-    [file:protected] => -
+    [file:protected] => %s
     [line:protected] => %d
     [trace:Exception:private] => Array
         (