Heap overflow with zero-length TXT record

[fweimer]

In the glibc fork of the Hesiod, we recently fixed a heap-based buffer overflow which is triggered by an invalid (zero-length) TXT record:

• https://sourceware.org/bugzilla/show_bug.cgi?id=20031

We did not treat this as a security vulnerability because Hesiod treats DNS data as trusted by design, so no trust boundary is crossed.