After resizing the scan buffer, we can immediately read into the
newly-resized buffer since we know there is now space.

Fixes #15712.

Change-Id: I56fcfaeb67045ee753a012c37883aa7c81b6e877
Reviewed-on: https://go-review.googlesource.com/31715
Run-TryBot: Quentin Smith <quentin@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>