p can be nil in Dconv so we need to do a check before dereferencing
it. Fixes a problem I was having running toolstash.

Change-Id: I34d6d278b319583d8454c2342ac88e054fc4b641
Reviewed-on: https://go-review.googlesource.com/20595Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>