cmd/cl: defend against msg.Author == nil
cmd/cl: defend against msg.Author == nil

Fixes crash on current Gerrit data.

Change-Id: I2097fa66afcbf938f880c89891d7d9ed65d843ac
Reviewed-on: https://go-review.googlesource.com/12555
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/build,evandbrown/build,evandbrown/build,proppy/build,evandbrown/build,proppy/build,proppy/build,golang/build,golang/build,golang/build,proppy/build