http2: fix incorrect panic
http2: fix incorrect panic

Previously, we panic'd on sending WINDOW_UPDATE on the half-closed-local
state. However, the RFC allows sending WINDOW_UPDATE in this state, so
we should no panic.

Change-Id: I702b2d5ad525837d7b8c650b7a2ed3f16371be63
Reviewed-on: https://go-review.googlesource.com/34498
Run-TryBot: Tom Bergan <3b77d7da34779f829901a664ac7c87d255b041af@google.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/net