PR 48587 Newunit allocator, cleanup

Improve error message, and remove a redundant check, as the same check is
done a bit earlier due to the PR 48587 patch.

2016-10-16  Janne Blomqvist  <jb@gcc.gnu.org>

        PR libfortran/48587
        * io/transfer.c (data_transfer_init): Improve error message,
        remove redundant check.

Regtested on x86_64-pc-linux-gnu.

From-SVN: r241211