[analyzer] CStringChecker buffer access checks should check the first bytes

By not checking if the first byte of the buffer is accessible,
we missed some reports in the Juliet benchmark.

(Juliet CWE-124 Buffer Underwrite: memcpy, memmove)
https://discourse.llvm.org/t/patches-inspired-by-the-juliet-benchmark/73106

Depends on D159108

Differential Revision: https://reviews.llvm.org/D159109