log-services: fix clang-tidy warnings

A number of similar warnings about unsafe pointer arithmetic.

```
../redfish-core/lib/log_services.hpp:269:39: error: unsafe pointer arithmetic [-Werror,-Wunsafe-buffer-usage]
            indexStr.data(), indexStr.data() + indexStr.size(), index);
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Icc4a0d2f418c76d6987ef2318b0098d30d116389