Plug a mem leak in the readwrite test

Plug a mem leak in the readwrite test

And use g_assert_[no_]error().

==2392== 49 (24 direct, 25 indirect) bytes in 1 blocks are definitely lost in loss record 451 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 0x403A8A6: g_error_new_literal (gerror.c:117)
==2392==    by 0x403AC31: g_set_error_literal (gerror.c:314)
==2392==    by 0x41B7619: g_output_stream_set_pending (goutputstream.c:1198)
==2392==    by 0x41B5799: g_output_stream_write (goutputstream.c:210)
==2392==    by 0x41B590B: g_output_stream_write_all (goutputstream.c:268)
==2392==    by 0x8049B54: verify_iostream (readwrite.c:110)

Bug #628331.