BUG/MEDIUM: dns: Don't prevent reading the last byte of the payload in dns_validate_response()

A regression was introduced with efbbdf72 BUG: dns: Prevent out-of-bounds
read in dns_validate_dns_response() as it prevented from taking into account
the last byte of the payload.  this patch aims at fixing it.

this must be backported in 1.8.
diff --git a/src/dns.c b/src/dns.c
index c1396f5..78d8f52 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -810,7 +810,7 @@
 		/* Move forward 2 bytes for data len */
 		reader += 2;
 
-		if (reader + dns_answer_record->data_len >= bufend) {
+		if (reader + dns_answer_record->data_len > bufend) {
 			pool_free(dns_answer_item_pool, dns_answer_record);
 			return DNS_RESP_INVALID;
 		}