bpo-42146: Fix memory leak in subprocess.Popen() in case of uid/gid overflow (GH-22966)

Fix memory leak in subprocess.Popen() in case of uid/gid overflow

Also add a test that would catch this leak with `--huntrleaks`.

Alas, the test for `extra_groups` also exposes an inconsistency
in our error reporting: we use a custom ValueError for `extra_groups`,
but propagate OverflowError for `user` and `group`.
diff --git a/Modules/_posixsubprocess.c b/Modules/_posixsubprocess.c
index 8baea31..5e5fbb2 100644
--- a/Modules/_posixsubprocess.c
+++ b/Modules/_posixsubprocess.c
@@ -772,7 +772,7 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
     uid_t uid;
     gid_t gid, *groups = NULL;
     int child_umask;
-    PyObject *cwd_obj, *cwd_obj2;
+    PyObject *cwd_obj, *cwd_obj2 = NULL;
     const char *cwd;
     pid_t pid;
     int need_to_reenable_gc = 0;
@@ -894,7 +894,6 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
         cwd = PyBytes_AsString(cwd_obj2);
     } else {
         cwd = NULL;
-        cwd_obj2 = NULL;
     }
 
     if (groups_list != Py_None) {
@@ -1080,6 +1079,7 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
     return PyLong_FromPid(pid);
 
 cleanup:
+    Py_XDECREF(cwd_obj2);
     if (envp)
         _Py_FreeCharPArray(envp);
     if (argv)