CamelGpgContext: Don't leak istream

The istream in gpg_sign_sync is ref'ed by the context so we need to release the ref we have.

This was found by looking at chatty's¹ pgp test under ASAN:

  meson setup -Db_sanitize=address _build
  meson test -C _build pgp --verbose

¹) https://gitlab.gnome.org/World/Chatty/-/blob/main/tests/pgp.c