Plug a mem leak in the readwrite test

Plug a mem leak in the readwrite test

==2392== 38 (16 direct, 22 indirect) bytes in 1 blocks are definitely lost in loss record 369 of 573
==2392==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==2392==    by 0x4057094: g_malloc (gmem.c:134)
==2392==    by 0x406F2D6: g_slice_alloc (gslice.c:836)
==2392==    by 0x406F31B: g_slice_alloc0 (gslice.c:848)
==2392==    by 0x413D5BB: g_type_create_instance (gtype.c:1867)
==2392==    by 0x412372A: g_object_constructor (gobject.c:1482)
==2392==    by 0x4122E1D: g_object_newv (gobject.c:1266)
==2392==    by 0x4122B93: g_object_new (gobject.c:1178)
==2392==    by 0x4225D74: _g_local_file_new (glocalfile.c:310)
==2392==    by 0x4231897: g_local_vfs_get_file_for_path (glocalvfs.c:84)
==2392==    by 0x41CF91C: g_vfs_get_file_for_path (gvfs.c:94)
==2392==    by 0x41911B6: g_file_new_for_path (gfile.c:5898)
==2392==    by 0x804A2B9: test_g_file_replace_readwrite (readwrite.c:235)

Bug #628331.