ptx: fix infinite loop with -G and a narrow output width

ptx: fix infinite loop with -G and a narrow output width

With GNU extensions disabled, before_max_width can go negative, and
define_all_fields then loops forever trying to shrink the before field
to a width narrower than empty:

  $ printf 'qux\n' | src/ptx -G -w2
  [hangs]

Commit v6.12-96-g773be9eca added a clamp for this, but placed it in
the GNU-extensions branch only.  Move it after that if/else so that
both branches are covered.

* src/ptx.c (fix_output_parameters): Clamp before_max_width for both
the GNU and non-GNU cases.
* tests/ptx/ptx-overrun.sh: Add a test case.
* NEWS: Mention the bug fix.
Fixes https://bugs.gnu.org/81507