[analyzer] CStringChecker: Fix crash in `CheckOverlap` when arguments are not pointers (#160511)
https://github.com/llvm/llvm-project/blob/main/clang/lib/StaticAnalyzer/Checkers/CStringChecker.cpp#L675-L678
mistakenly assumes that target expressions are of pointer type.
`CheckOverlap` has multiple call sites, most of which do not verify this
assumption. Therefore, the simplest solution is to verify it just before
that point.