cmd/godoc: synchronously init corpus for index mode

The corpus needs to be init'd before generating the index.

Regression caused by CL 141397.

Updates golang/go#25443.

Change-Id: I04d19bf5d279031cc8ddf0f850d81eb06fc5efa8
Reviewed-on: https://go-review.googlesource.com/c/141717
Run-TryBot: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/godoc/main.go

@@ -241,8 +241,10 @@ func main() {
 	if *writeIndex {
 		corpus.IndexThrottle = 1.0
 		corpus.IndexEnabled = true
-	}
+		initCorpus(corpus)
+	} else {
 		go initCorpus(corpus)
+	}
 
 	// Initialize the version info before readTemplates, which saves
 	// the map value in a method value.