SECURITY: Do not overrun reading buffer if domain ends with backslash

SECURITY: Do not overrun reading buffer if domain ends with backslash

If the query domain ended with \, and adns_qf_quoteok_query was
specified, qdparselabel would read additional bytes from the buffer
and try to treat them as the escape sequence.  It would depart the
input buffer and start processing many bytes of arbitrary heap data as
if it were the query domain.

Eventually it would run out of input or find some other kind of error,
and declare the query domain invalid.  But before then it might outrun
available memory and crash.

In principle this could be a denial of service attack.

Found by AFL 2.35b.  CVE-2017-9107.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>