Plug a mem leak in data-input-stream test

Plug a mem leak in data-input-stream test

==2415== 45,045 bytes in 4,095 blocks are definitely lost in loss record 393 of 399
==2415==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==2415==    by 0x4057094: g_malloc (gmem.c:134)
==2415==    by 0x417FC29: g_data_input_stream_read_line (gdatainputstream.c:797)
==2415==    by 0x8049874: test_read_lines (data-input-stream.c:111)

==12088== 360 bytes in 40 blocks are definitely lost in loss record 368 of 381
==12088==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==12088==    by 0x4057094: g_malloc (gmem.c:134)
==12088==    by 0x417FF4C: g_data_input_stream_read_until (gdatainputstream.c:914)
==12088==    by 0x8049B6F: test_read_until (data-input-stream.c:182)

Bug #628331.