path: handle error returned by `git_buf_joinpath`

In the `_check_dir_contents` function, we first allocate memory for
joining the directory and subdirectory together and afterwards use
`git_buf_joinpath`. While this function in fact should not fail as
memory is already allocated, err on the safe side and check for returned
errors.

src/path.c

@@ -700,7 +700,8 @@ static bool _check_dir_contents(
 		return false;
 
 	/* save excursion */
-	git_buf_joinpath(dir, dir->ptr, sub);
+	if (git_buf_joinpath(dir, dir->ptr, sub) < 0)
+		return false;
 
 	result = predicate(dir->ptr);