Plug a mem leak in buffered-input-stream test

Plug a mem leak in buffered-input-stream test

==2429== 49 (24 direct, 25 indirect) bytes in 1 blocks are definitely lost in loss record 276 of 355
==2429==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==2429==    by 0x4057094: g_malloc (gmem.c:134)
==2429==    by 0x406F2D6: g_slice_alloc (gslice.c:836)
==2429==    by 0x406F31B: g_slice_alloc0 (gslice.c:848)
==2429==    by 0x403A8A6: g_error_new_literal (gerror.c:117)
==2429==    by 0x403AC31: g_set_error_literal (gerror.c:314)
==2429==    by 0x4175525: g_buffered_input_stream_read_byte (gbufferedinputstream.c:880)
==2429==    by 0x804A21A: test_read_byte (buffered-input-stream.c:153)

Bug #628331.