[m-rev.] for review: Fix crash in mercury_close in non-gc C grades.

[Julien Fischer]

On Wed, 21 Aug 2019, Peter Wang wrote:

> library/io.m:
>    In mercury_open(), allocate a MercuryFile with MR_GC_NEW_ATTRIB
>    instead of MR_incr_hp_type_msg to match the use of MR_GC_free
>    in mercury_close() to free that allocation.

That's fine.

Julien.