gh-120321: Add missing "return false" in gen_try_set_executing by colesbury · Pull Request #144291 · python/cpython

…ythongh-144291)

We didn't catch this because of a combination of:

1) falling through to the if-statement below works
2) we only specialized FOR_ITER_GEN for uniquely referenced generators,
   so we didn't trigger the non-thread-safe behavior.