cpython3:fuzz_builtin_unicode: Use-of-uninitialized-value in maybe_small_long

There is a bug disclosed by oss-fuzz: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=51574.

I can reproduce it by running `CC=clang ./configure --with-memory-sanitizer && make -j12`.


### Linked PRs
* gh-102510
* gh-102838
* gh-107464