CLEANUP: dns: remove duplicated code in dns_validate_dns_response()

a reader pointer comparison to the end of the buffer was performed twice
while once is obviously enough.

backport status: this patch can be backported into HAProxy 1.6 (with some
modification. Please contact me)
diff --git a/src/dns.c b/src/dns.c
index 82c7329..421beab 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -1239,11 +1239,6 @@
 			return DNS_RESP_INVALID;
 		}
 
-		if (reader >= bufend) {
-			free_dns_answer_item(dns_answer_record);
-			return DNS_RESP_INVALID;
-		}
-
 		/* 2 bytes for record type (A, AAAA, CNAME, etc...) */
 		if (reader + 2 > bufend) {
 			free_dns_answer_item(dns_answer_record);