cmd/buildlet: fix reverse check
cmd/buildlet: fix reverse check

Change-Id: Iac6f38aa25808e4c41c469b178c5d402a836e545
Reviewed-on: https://go-review.googlesource.com/39233
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build