Updating to commit fffc5831a499a958516664a34cb7ba2b9e228793
from github.com/google/pprof

Fixes #19380

Change-Id: I7a0c64101f42b494c4a469c41628374272eccf95
Reviewed-on: https://go-review.googlesource.com/46155
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>