gh-151673: Fix crash in warnings.warn() under memory pressure

[Ijtihed]

When warnings.warn() runs while every allocation is failing setup_context() took the f == NULL branch and called PyUnicode_FromString("<sys>") without checking the result. The resulting NULL filename was then passed to Py_DECREF either by do_warn() on the success path or by setup_context()'s own handle_error: label (which used Py_DECREF, not Py_XDECREF). This caused a segfault.

The fix initialize the output references up front then check the "<sys>" allocation and use Py_XDECREF in the error path.

Fixes#151673

• Issue: Segfault: Py_DECREF(NULL) in setup_context/do_warn (_warnings.c) when emitting a warning under MemoryError #151673

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[Ijtihed]

the ci failure was preexisting on main. i can update branch when needed

[Ijtihed]

r? @StanFromIreland