BUG/MINOR: hbuf: treat unexpected escape sequences as literals

BUG/MINOR: hbuf: treat unexpected escape sequences as literals

When encountering an unexpected escape sequence, treat it as a literal
character instead of skipping it.

This is a deliberate choice for two reasons:

- to avoid a desynchronization between the h->data counter and the
  buffer content, which would otherwise leave uninitialized memory
  ("garbage") in the destination buffer;

- to ensure that an invalid configuration string triggers a parsing
  error immediately (fail-fast), rather than resulting in a silently
  malformed configuration.

Thank to @dirkmueller for having reported this issue.

No need to backport. huf arrived with this current version.