[Debug] Fixed recursion level incrementing in FlattenException::flattenArgs().

The internal `$level` variable for tracking the recursion level is pre-incremented on the parent level of the recursion already.

This causes later array elements in an array that has more than 10 elements to get obscured by `'*DEEP NESTED ARRAY*'`, even though the elements are on the first/top level of the array.

The incremented `$level` value needs to be passed to the recursive call to `FlattenException::flattenArgs()` only.

Discovered in debugging exceptions in Drupal (which happens to use very large multi-dimensional arrays for legacy reasons).

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -