Plug a mem leak in converter-stream test

Plug a mem leak in converter-stream test

==2396== 66 (24 direct, 42 indirect) bytes in 1 blocks are definitely lost in loss record 565 of 625
==2396==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==2396==    by 0x4057094: g_malloc (gmem.c:134)
==2396==    by 0x406F2D6: g_slice_alloc (gslice.c:836)
==2396==    by 0x406F31B: g_slice_alloc0 (gslice.c:848)
==2396==    by 0x403A8A6: g_error_new_literal (gerror.c:117)
==2396==    by 0x403AC31: g_set_error_literal (gerror.c:314)
==2396==    by 0x417BA38: g_charset_converter_convert (gcharsetconverter.c:344)
==2396==    by 0x417BF67: g_converter_convert (gconverter.c:174)
==2396==    by 0x417C9EB: g_converter_input_stream_read (gconverterinputstream.c:403)
==2396==    by 0x41A7A17: g_input_stream_read (ginputstream.c:204)
==2396==    by 0x41A7B43: g_input_stream_read_all (ginputstream.c:256)
==2396==    by 0x804B0E4: test_charset (converter-stream.c:682)

Bug #628331.