Start gather goroutine before looping over Scanner.

Otherwise gather is blocked when the workers try to write to it after
processing their first elements since nothing is reading from it, yet.

Thanks to Sean Liao on the Gophers Slack for pointing this out.
This resolves blackhat-go#11.

ch-5/subdomain_guesser/main.go

@@ -119,11 +119,6 @@ func main() {
 		go worker(tracker, fqdns, gather, *flServerAddr)
 	}
 
-	for scanner.Scan() {
-		fqdns <- fmt.Sprintf("%s.%s", scanner.Text(), *flDomain)
-	}
-	// Note: We could check scanner.Err() here.
-
 	go func() {
 		for r := range gather {
 			results = append(results, r...)
@@ -132,6 +127,11 @@ func main() {
 		tracker <- e
 	}()
 
+	for scanner.Scan() {
+		fqdns <- fmt.Sprintf("%s.%s", scanner.Text(), *flDomain)
+	}
+	// Note: We could check scanner.Err() here.
+
 	close(fqdns)
 	for i := 0; i < *flWorkerCount; i++ {
 		<-tracker