[3.13] gh-121652: Handle `allocate_weakref` returning NULL (GH-121653) (#121721)

The `allocate_weakref` may return NULL when out of memory. We need to
handle that case and propagate the error.
(cherry picked from commit a640a605a8a1a3f73b98f948d0c2a7d42134f692)

Co-authored-by: Sam Gross <colesbury@gmail.com>