x/build/maintner/maintnerd: check CL is found before doing work
x/build/maintner/maintnerd: check CL is found before doing work

Fixes golang/go#21383

Change-Id: I58582709fd83bb5c6ee452bdc7e53658f58dcad8
Reviewed-on: https://go-review.googlesource.com/54751
Reviewed-by: Yuval Pavel Zholkover <170f3b94ed768c88b460d7f0c362c1d8ffaf0cec@gmail.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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