cmd/godoc: synchronously init corpus for index mode
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 <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools