Resolver: explicit check for compression pointers in question.

Since nginx always uses exactly one entry in the question section of
a DNS query, and never uses compression pointers in this entry, parsing
of a DNS response in ngx_resolver_process_response() does not expect
compression pointers to appear in the question section of the DNS
response.  Indeed, compression pointers in the first name of a DNS response
hardly make sense, do not seem to be allowed by RFC 1035 (which says
"a pointer to a prior occurance of the same name", note "prior"), and
were never observed in practice.

Added an explicit check to ngx_resolver_process_response()'s parsing
of the question section to properly report an error if compression pointers
nevertheless appear in the question section.
diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index 9ce53b9..58d5f3e 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -1798,6 +1798,12 @@
     i = sizeof(ngx_resolver_hdr_t);
     while (i < (ngx_uint_t) n) {
+        if (buf[i] & 0xc0) {
+            err = "unexpected compression pointer in DNS response";
+            goto done;
+        }
         if (buf[i] == '\0') {
             goto found;