Fix uninitialized variable when looking up '_'.
Fix uninitialized variable when looking up '_'.

R=iant
CC=gofrontend-dev
https://golang.org/cl/3753043

anlhord/gofrontend,golang/gofrontend,golang/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,anlhord/gofrontend,golang/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,golang/gofrontend,anlhord/gofrontend,anlhord/gofrontend,qskycolor/gofrontend