repo/commit: Fix memory leak

repo/commit: Fix memory leak

While running the testsuite under valgrind a small memory leak showed up:

==16487== 65 bytes in 1 blocks are definitely lost in loss record 773 of 1,123
==16487==    at 0x4C2BBAF: malloc (vg_replace_malloc.c:299)
==16487==    by 0x6048E08: g_malloc (gmem.c:94)
==16487==    by 0x6062EAE: g_strdup (gstrfuncs.c:363)
==16487==    by 0x54CE3E6: write_object (ostree-repo-commit.c:776)
==16487==    by 0x54CF2D4: ostree_repo_write_metadata (ostree-repo-commit.c:1528)
==16487==    by 0x54CF505: _ostree_repo_write_directory_meta (ostree-repo-commit.c:1712)
==16487==    by 0x54D0AB4: write_dfd_iter_to_mtree_internal (ostree-repo-commit.c:2650)
==16487==    by 0x54D0E2D: ostree_repo_write_dfd_to_mtree (ostree-repo-commit.c:2793)
==16487==    by 0x1190C4: ostree_builtin_commit (ot-builtin-commit.c:474)
==16487==    by 0x11F2EE: ostree_run (ot-main.c:200)
==16487==    by 0x116F32: main (main.c:78)

The reason for this is that ot_checksum_instream_get_string returns a chunk of newly allocated memory which never got freed.

Make actual_checksum something that gets autocleanend and own the memory
assigned to it in all cases.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Closes: #827
Approved by: pwithnall