Fix write_object.

Two bugs:
1. zstream free.
2. write_object free.

```
(gdb) bt
#0  0x00007fff8f4e9d46 in __kill ()
#1  0x00007fff8d0d6f83 in abort ()
#2  0x00007fff8d0aa989 in free ()
#3  0x000000010051d869 in git__free [inlined] () at /Users/xtao/Work/code/pygit2_xt/vendor/libgit2/src/util.h:354
#4  0x000000010051d869 in write_one (status=<value temporarily unavailable, due to optimizations>, pb=0x100376a10, po=0x10077ea28, write_cb=0x100527b00 <foreach_cb>, cb_data=0x7fff5fbfe8f0) at src/pack-objects.c:354
#5  0x000000010051ece0 in write_pack (pb=0x100376a10, write_cb=0x100527b00 <foreach_cb>, cb_data=0x7fff5fbfe8f0) at src/pack-objects.c:620
#6  0x0000000100527df7 in local_download_pack (transport=0x10035cda0, repo=0x100340520, stats=0x100366718, progress_cb=0, progress_payload=0x0) at src/transports/local.c:540
#7  0x000000010050ed75 in git_remote_download (remote=0x100366640) at src/remote.c:827
#8  0x00000001004ba94a in Remote_fetch ()
#9  0x00000001000ec591 in call_function ()
#10 0x00000001000e8bbd in PyEval_EvalFrameEx ()
#11 0x00000001000eaa6f in PyEval_EvalCodeEx ()
#12 0x00000001000e1f37 in PyEval_EvalCode ()
#13 0x00000001001190ef in run_mod ()
#14 0x0000000100119079 in PyRun_FileExFlags ()
#15 0x0000000100117dd3 in PyRun_SimpleFileExFlags ()
#16 0x000000010011766e in PyRun_AnyFileExFlags ()
#17 0x0000000100133b85 in Py_Main ()
#18 0x0000000100000ead in main ()
```