bufio: use underlying ReadFrom even when data is buffered
When (*bufio.Writer).ReadFrom is called with a partially filled buffer,
fill out and flush the buffer and then call the underlying writer's
ReadFrom method if present.
Fixes #44815.
Change-Id: I15b3ef0746d0d60fd62041189a9b9df11254dd29
Reviewed-on: https://go-review.googlesource.com/c/go/+/340530
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>