gh-130070: Fix exec(<string>, closure=<non-None>) unexpected path (#130071)

Fixed an assertion error (so, it could be reproduced only in builds with assertions enabled)
for `exec` when the `source` argument is a string and the `closure` argument is not `None`.

Co-authored-by: sobolevn <mail@sobolevn.me>

Python/bltinmodule.c

@@ -1169,6 +1169,7 @@ builtin_exec_impl(PyObject *module, PyObject *source, PyObject *globals,
         if (closure != NULL) {
             PyErr_SetString(PyExc_TypeError,
                 "closure can only be used when source is a code object");
+            goto error;
         }
         PyObject *source_copy;
         const char *str;