[3.12] Fix possible null pointer dereference of freevars in _PyCompile_LookupArg (gh-126238) (#126474)

[3.12] gh-126238: Fix possible null pointer dereference of freevars in _PyCompile_LookupArg (GH-126239)

* Replace Py_DECREF by Py_XDECREF

(cherry picked from commit 8525c9375f25e6ec0c0b5dfcab464703f6e78082)

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Peter Bierma <zintensitydev@gmail.com>