[FunctionCallGraph] Stack use-after-scope reported by Address sanitizer

[bshastry]

Likely introduced by

See #10973 (comment) and https://github.com/google/oss-fuzz/pull/5701/checks?check_run_id=2492467934#step:8:21763

The fuzzer build is failing due to this.

[cameel]

I've been meaning to get back to this one when I have a bit more time to dig deeper. I thought this was low priority but if it's actually a fuzz blocker then my suggestion would be just to create a local and use it instead of a temporary in valueOrDefault().