Plug a mem leak in convert-stream test

Plug a mem leak in convert-stream test

==7540== 487 (64 direct, 423 indirect) bytes in 2 blocks are definitely lost in loss record 597 of 615
==7540==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==7540==    by 0x4057094: g_malloc (gmem.c:134)
==7540==    by 0x406F2D6: g_slice_alloc (gslice.c:836)
==7540==    by 0x406F31B: g_slice_alloc0 (gslice.c:848)
==7540==    by 0x413D5BB: g_type_create_instance (gtype.c:1867)
==7540==    by 0x412372A: g_object_constructor (gobject.c:1482)
==7540==    by 0x4123147: g_object_newv (gobject.c:1347)
==7540==    by 0x41236BB: g_object_new_valist (gobject.c:1463)
==7540==    by 0x41A756E: g_initable_new_valist (ginitable.c:214)
==7540==    by 0x41A743E: g_initable_new (ginitable.c:138)
==7540==    by 0x417B67A: g_charset_converter_new (gcharsetconverter.c:215)
==7540==    by 0x804B043: test_charset (converter-stream.c:675)

Bug #628331.