[clang] [analyzer] CStringChecker: fix crash in `CheckOverlap` when arguments are not pointers (PR #160511)

[Donát Nagy via cfe-commits]

https://github.com/NagyDonat approved this pull request.

Thanks for the commit, crash fixes are always welcome!

I added two very minor stylistic suggestion in inline comments, but the commit is also acceptable in its current form.

Moreover, in the commit message you write that "`CheckOverlap` has multiple call sites, most of which do not verify this assumption." which implies that some call sites _did_ verify this assumption. Now that `CheckOverlap` does this verification, are there any call sites where the verification logic is made completely redundant by your change?

https://github.com/llvm/llvm-project/pull/160511