BUG/MINOR: resolvers: validate TXT RDATA before reading

BUG/MINOR: resolvers: validate TXT RDATA before reading

maxml reported on the public HAProxy mailing list that the DNS response
validator reads the first TXT character-string length byte before checking
that the record contains any RDATA.

When a TXT answer has RDLENGTH zero and ends exactly at bufend, the generic
RDATA bounds check accepts the empty range. The TXT parser then reads
reader[0] with reader == bufend, before rejecting the record. Short-circuit
evaluation of the subsequent condition does not protect this earlier read.

The report includes a minimized 31-byte DNS response which triggers an
AddressSanitizer one-byte heap-buffer-overflow with an exactly sized
libFuzzer input. All 13 original AFL++ crash inputs reach the same read.

The production caller uses a DNS_MAX_MSG_SIZE + 1 stack buffer and parses
at most DNS_MAX_MSG_SIZE bytes, so this read remains within the allocation.
The loaded value is discarded when the zero-length check fails. No
production crash or information disclosure was demonstrated.

Check that RDATA contains at least one byte before reading its first
character-string length. A valid empty TXT string remains accepted: its
RDLENGTH is one and its character-string length byte is zero.

This was introduced by e418e828aa ("MINOR: resolvers: basic TXT record
implementation"). It should be backported to 3.4. Older stable branches
are not affected.