Some object-to-AST conversions are missing error checks

The generated code in Python-ast.c is missing error checks following the construction of C-level alias, arg, comprehension, keyword, match_item, and withitem nodes from their Python object counterparts. This means it's possible to crash the interpreter by attempting to compile an AST where a required member of these nodes is replaced with None:

>>> import ast
>>> tree = ast.parse("""
... match ...:
...     case THIS:
...         ...
... """)
>>> tree.body[0].cases[0].pattern = None
>>> compile(tree, "<crash>", "exec")
Segmentation fault

I'll have a PR up in a minute with the one-line fix.

Linked PRs

• GH-105588: Add missing error checks to some obj2ast_* converters #105589
• [3.12] GH-105588: Add missing error checks to some obj2ast_* converters (GH-105589) #105838
• [3.11] GH-105588: Add missing error checks to some obj2ast_* converters (GH-105589) #105839