[3.12] gh-126991: Fix reference leak in loading pickle's opcode BUILD (GH-126990) (GH-127019)

If PyObject_SetItem() fails in the `load_build()` function of _pickle.c, no DECREF for the `dict` variable.
(cherry picked from commit 29cbcbd73bbfd8c953c0b213fb33682c289934ff)

Co-authored-by: Justin Applegate <70449145+Legoclones@users.noreply.github.com>