compiler: Always initialize *nested_off in interpret_header.
compiler: Always initialize *nested_off in interpret_header.

http://gcc.gnu.org/PR56198

R=iant
CC=gofrontend-dev
https://golang.org/cl/7299043

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