submodule: fix cwd leak in `get_superproject_working_tree()`

submodule: fix cwd leak in `get_superproject_working_tree()`

`get_superproject_working_tree()` allocates cwd via `xgetcwd()` at the
top of the function, but two early-return paths (when not inside a work
tree, and when strbuf_realpath for "../" fails) return 0 without freeing
it.

Redirect these early returns through a cleanup label that frees cwd
before returning.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>