compiler: Don't crash when dumping ast of empty block.
compiler: Don't crash when dumping ast of empty block.

Fixes golang/go#10420.

Change-Id: Ie8d7209cda36aff975bd02b0d1086d92883c069c
Reviewed-on: https://go-review.googlesource.com/10650
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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