index: convert `read_entry` to return entry size via an out-param

The function `read_entry` does not conform to our usual coding style of
returning stuff via the out parameter and to use the return value for
reporting errors. Due to most of our code conforming to that pattern, it
has become quite natural for us to actually return `-1` in case there is
any error, which has also slipped in with commit 5625d86b9 (index:
support index v4, 2016-05-17). As the function returns an `size_t` only,
though, the return value is wrapped around, causing the caller of
`read_tree` to continue with an invalid index entry. Ultimately, this
can lead to a double-free.

Improve code and fix the bug by converting the function to return the
index entry size via an out parameter and only using the return value to
indicate errors.

Reported-by: Krishna Ram Prakash R <krp@gtux.in>
Reported-by: Vivek Parikh <viv0411.parikh@gmail.com>
static int index_apply_to_wd_diff(git_index *index, int action, const git_strarray *paths,
				  unsigned int flags,
				  git_index_matched_path_cb cb, void *payload)
{
	int error;
	git_diff *diff;
	git_pathspec ps;
	git_repository *repo;
	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
	struct foreach_diff_data data = {
		index,
		NULL,
		flags,
		cb,
		payload,
	};

	assert(index);
	assert(action == INDEX_ACTION_UPDATE || action == INDEX_ACTION_ADDALL);

	repo = INDEX_OWNER(index);

	if (!repo) {
		return create_index_error(-1,
			"cannot run update; the index is not backed up by a repository.");
	}

	/*
	 * We do the matching ourselves intead of passing the list to
	 * diff because we want to tell the callback which one
	 * matched, which we do not know if we ask diff to filter for us.
	 */
	if ((error = git_pathspec__init(&ps, paths)) < 0)
		return error;

	opts.flags = GIT_DIFF_INCLUDE_TYPECHANGE;
	if (action == INDEX_ACTION_ADDALL) {
		opts.flags |= GIT_DIFF_INCLUDE_UNTRACKED |
			GIT_DIFF_RECURSE_UNTRACKED_DIRS;

		if (flags == GIT_INDEX_ADD_FORCE)
			opts.flags |= GIT_DIFF_INCLUDE_IGNORED;
	}

	if ((error = git_diff_index_to_workdir(&diff, repo, index, &opts)) < 0)
		goto cleanup;

	data.pathspec = &ps;
	error = git_diff_foreach(diff, apply_each_file, NULL, NULL, NULL, &data);
	git_diff_free(diff);

	if (error) /* make sure error is set if callback stopped iteration */
		giterr_set_after_callback(error);

cleanup:
	git_pathspec__clear(&ps);
	return error;
}

static int index_apply_to_wd_diff(git_index *index, int action, const git_strarray *paths,
				  unsigned int flags,
				  git_index_matched_path_cb cb, void *payload)
{
	int error;
	git_diff *diff;
	git_pathspec ps;
	git_repository *repo;
	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
	struct foreach_diff_data data = {
		index,
		NULL,
		flags,
		cb,
		payload,
	};

	assert(index);
	assert(action == INDEX_ACTION_UPDATE || action == INDEX_ACTION_ADDALL);

	repo = INDEX_OWNER(index);

	if (!repo) {
		return create_index_error(-1,
			"cannot run update; the index is not backed up by a repository.");
	}

	/*
	 * We do the matching ourselves intead of passing the list to
	 * diff because we want to tell the callback which one
	 * matched, which we do not know if we ask diff to filter for us.
	 */
	if ((error = git_pathspec__init(&ps, paths)) < 0)
		return error;

	opts.flags = GIT_DIFF_INCLUDE_TYPECHANGE;
	if (action == INDEX_ACTION_ADDALL) {
		opts.flags |= GIT_DIFF_INCLUDE_UNTRACKED |
			GIT_DIFF_RECURSE_UNTRACKED_DIRS;

		if (flags == GIT_INDEX_ADD_FORCE)
			opts.flags |= GIT_DIFF_INCLUDE_IGNORED;
	}

	if ((error = git_diff_index_to_workdir(&diff, repo, index, &opts)) < 0)
		goto cleanup;

	data.pathspec = &ps;
	error = git_diff_foreach(diff, apply_each_file, NULL, NULL, NULL, &data);
	git_diff_free(diff);

	if (error) /* make sure error is set if callback stopped iteration */
		giterr_set_after_callback(error);

cleanup:
	git_pathspec__clear(&ps);
	return error;
}
