Plug some huge mem leaks in converter-stream test

Plug some huge mem leaks in converter-stream test

==8564== 24,000,000 bytes in 6 blocks are possibly lost in loss record 592 of 594
==8564==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==8564==    by 0x4057094: g_malloc (gmem.c:134)
==8564==    by 0x804AA37: test_corruption (converter-stream.c:589)
==8564==    by 0x804B05B: test_roundtrip (converter-stream.c:652)

==9459== 25,165,824 bytes in 6 blocks are possibly lost in loss record 593 of 594
==9459==    at 0x4005CD2: realloc (vg_replace_malloc.c:476)
==9459==    by 0x40571A5: g_realloc (gmem.c:181)
==9459==    by 0x41B08A3: array_resize (gmemoryoutputstream.c:501)
==9459==    by 0x41B0A5D: g_memory_output_stream_write (gmemoryoutputstream.c:578)
==9459==    by 0x41B57EF: g_output_stream_write (goutputstream.c:216)
==9459==    by 0x41B591B: g_output_stream_write_all (goutputstream.c:268)
==9459==    by 0x417D617: flush_buffer (gconverteroutputstream.c:359)
==9459==    by 0x417D958: g_converter_output_stream_write (gconverteroutputstream.c:502)
==9459==    by 0x41B5D7F: g_output_stream_real_splice (goutputstream.c:428)
==9459==    by 0x41B5C6C: g_output_stream_splice (goutputstream.c:380)
==9459==    by 0x804AB10: test_corruption (converter-stream.c:600)

==9785== 25,165,824 bytes in 6 blocks are possibly lost in loss record 592 of 592
==9785==    at 0x4005CD2: realloc (vg_replace_malloc.c:476)
==9785==    by 0x40571A5: g_realloc (gmem.c:181)
==9785==    by 0x41B08A3: array_resize (gmemoryoutputstream.c:501)
==9785==    by 0x41B0A5D: g_memory_output_stream_write (gmemoryoutputstream.c:578)
==9785==    by 0x41B5D7F: g_output_stream_real_splice (goutputstream.c:428)
==9785==    by 0x41B5C6C: g_output_stream_splice (goutputstream.c:380)
==9785==    by 0x804ADF1: test_corruption (converter-stream.c:622)
==9785==    by 0x804B06C: test_roundtrip (converter-stream.c:652)

Bug #628331.