When a reserved name (e.g. __classdict__) is used as a type parameter,
symtable_visit_type_param_bound_or_default() calls PyUnicode_FromFormat()
to build the SyntaxError message. If the allocation fails and returns NULL,
the subsequent PyErr_SetObject(PyExc_SyntaxError, NULL) and Py_DECREF(NULL)
calls dereference the null pointer, causing a segfault.

Fix by returning 0 immediately when PyUnicode_FromFormat() returns NULL,
which propagates the MemoryError already set by PyUnicode_FromFormat().

The bug was introduced in gh-128632 (commit 891c61c).

Found by static analyzer Svace (ISP RAS).

• Add a NEWS entry in Misc/NEWS.d/
• Add regression test (test_disallowed_type_param_names_oom)

• Issue: symtable: NULL dereference after OOM in PyUnicode_FromFormat (symtable_visit_type_param_bound_or_default) #152682