x/tools/refactor/rename: fix nil pointer dereference.

Fix a nil pointer dereference by checking the error returned by
conf.Load.

Bug was reported on Reddit:
https://www.reddit.com/r/golang/comments/5vo0fb/

Change-Id: I2ad6f03922b772f03e282a0af6eea24f8de9cef8
Reviewed-on: https://go-review.googlesource.com/37432
Reviewed-by: Alan Donovan <adonovan@google.com>
2 files changed