libgo/runtime: return 0, not NULL, from main
libgo/runtime: return 0, not NULL, from main

Change-Id: I5f905bf31ac7a418a38f0f5817d65f72851120ac
Reviewed-on: https://go-review.googlesource.com/13421
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend