Avoid an out-of-range error when calling LongString on a generic
block.

Change-Id: I33ca88940d899bc71e3155bc63d2aa925cf83230
Reviewed-on: https://go-review.googlesource.com/c/go/+/200737
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>