Missing NULL check in AC *args tuple parsing

[kumaraditya303]

__clinic_args is currently not checked for NULL and can crash the interpreter. The fix is to check for NULL and exit early.

Linked PRs

• GH-100342: check for allocation failure in AC *args parsing #100343
• [3.11] GH-100342: check for allocation failure in AC *args parsing (GH-100343). #100568

[kumaraditya303]

cc @erlend-aasland