fru: Fix buffer overflow vulnerabilities

Partial fix for CVE-2020-5208, see
https://github.com/ipmitool/ipmitool/security/advisories/GHSA-g659-9qxw-p7cp

The `read_fru_area_section` function only performs size validation of
requested read size, and falsely assumes that the IPMI message will not
respond with more than the requested amount of data; it uses the
unvalidated response size to copy into `frubuf`. If the response is
larger than the request, this can result in overflowing the buffer.

The same issue affects the `read_fru_area` function.