mempack functions: return an int

Stop returning a void for functions, future-proofing them to allow them
to fail.

include/git2/sys/mempack.h

@@ -78,8 +78,9 @@ GIT_EXTERN(int) git_mempack_dump(git_buf *pack, git_repository *repo, git_odb_ba
  * semantics to the Git repository.
  *
  * @param backend The mempack backend
+ * @return 0 on success; error code otherwise
  */
-GIT_EXTERN(void) git_mempack_reset(git_odb_backend *backend);
+GIT_EXTERN(int) git_mempack_reset(git_odb_backend *backend);
 
 GIT_END_DECL
 

src/odb_mempack.c

@@ -125,7 +125,7 @@ cleanup:
 	return err;
 }
 
-void git_mempack_reset(git_odb_backend *_backend)
+int git_mempack_reset(git_odb_backend *_backend)
 {
 	struct memory_packer_db *db = (struct memory_packer_db *)_backend;
 	struct memobject *object = NULL;
@@ -137,6 +137,8 @@ void git_mempack_reset(git_odb_backend *_backend)
 	git_array_clear(db->commits);
 
 	git_oidmap_clear(db->objects);
+
+	return 0;
 }
 
 static void impl__free(git_odb_backend *_backend)