dns: suppress minor scan-build warnings

These were only used if debug is enabled.

app-layer-dns-tcp.c:407:13: warning: Value stored to 'length' is never read
            length = *data;
app-layer-dns-udp.c:236:13: warning: Value stored to 'length' is never read
            length = *data;

src/app-layer-dns-tcp.c

@@ -404,8 +404,7 @@ static int DNSReponseParseData(Flow *f, DNSState *dns_state, const uint8_t *inpu
                 goto insufficient_data;
             }
 
-            length = *data;
-            SCLogDebug("length %u", length);
+            SCLogDebug("length %u", *data);
         }
         if (fqdn_offset) {
             fqdn_offset--;

src/app-layer-dns-udp.c

@@ -233,8 +233,7 @@ static int DNSUDPResponseParse(Flow *f, void *dstate,
                 goto insufficient_data;
             }
 
-            length = *data;
-            SCLogDebug("length %u", length);
+            SCLogDebug("length %u", *data);
         }
         if (fqdn_offset) {
             fqdn_offset--;