[staticanalysis]: Actually check errors on splice() of objects

We were using unsigned size when we should have been using signed,
this means we basically weren't checking for errors on write...ouch.

Luckily if we e.g. hit ENOSPC during a pull, the checksums wouldn't
match and we'd return an error anyways.  However when writing an
object, we'd end up silently ignoring it =/

https://bugzilla.gnome.org/show_bug.cgi?id=732020

src/libostree/ostree-repo-commit.c

@@ -365,7 +365,7 @@ write_object (OstreeRepo         *self,
   gboolean temp_file_is_regular;
   gboolean is_symlink = FALSE;
   char loose_objpath[_OSTREE_LOOSE_PATH_MAX];
-  gsize unpacked_size = 0;
+  gssize unpacked_size = 0;
   gboolean indexable = FALSE;
 
   g_return_val_if_fail (expected_checksum || out_csum, FALSE);