test get_git_work_tree() return value for NULL

If we are in a git directory, get_git_work_tree() can return NULL.
While trying to determine whether or not the given paths are outside
the work tree, the following command would read from it anyways and
trigger a segmentation fault.

 git diff / /
Signed-off-by: NClemens Buchacher <drizzd@aon.at>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

diff-no-index.c

@@ -150,16 +150,14 @@ static int queue_diff(struct diff_options *o,
 
 static int path_outside_repo(const char *path)
 {
-	/*
-	 * We have already done setup_git_directory_gently() so we
-	 * know we are inside a git work tree already.
-	 */
 	const char *work_tree;
 	size_t len;
 
 	if (!is_absolute_path(path))
 		return 0;
 	work_tree = get_git_work_tree();
+	if (!work_tree)
+		return 1;
 	len = strlen(work_tree);
 	if (strncmp(path, work_tree, len) ||
 	    (path[len] != '\0' && path[len] != '/'))