core: add nil check (#1005)

Check if msg is nil in DefaultErrorFunc. If this is the case log this
and short cut the function.

Hoping to get more insight in #925

core/dnsserver/server.go

@@ -289,6 +289,12 @@ func DefaultErrorFunc(w dns.ResponseWriter, r *dns.Msg, rc int) {
 	answer := new(dns.Msg)
 	answer.SetRcode(r, rc)
 
+	if r == nil {
+		log.Printf("[WARNING] DefaultErrorFunc called with nil *dns.Msg (Remote: %s)", w.RemoteAddr().String())
+		w.WriteMsg(answer)
+		return
+	}
+
 	state.SizeAndDo(answer)
 
 	vars.Report(state, vars.Dropped, rcode.ToString(rc), answer.Len(), time.Now())