[clang] [-Wunsafe-buffer-usage] Fix crashes in "Add check for custom printf/scanf functions #173096" (PR #174683)

[Paul Osmialowski via cfe-commits]

pawosm-arm wrote:

> Is this ready to go, or if not could we revert the breaking change in the meantime?

It's getting closer to the release branching point, so we may need to...

https://github.com/llvm/llvm-project/pull/174683