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.

src/hbuf.c

@@ -55,6 +55,10 @@ void hbuf_str_append(struct hbuf *h, const char *line)
 			}
 			else if (*p == 't')
 				*to++ = '\t';
+			else {
+				/* unexpected escape sequence, treat as literal */
+				*to++ = *p;
+			}
 			p++;
 			h->data++;
 		}