go.tools/vcs: allow compilation with go 1.0

It would be nice to be able to use this package
as a dependency (or other go utilities in the
ecosystem that depend on this package) in
environments which have not (or cannot) for
whatever reason upgraded to newer versions of
golang.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/14283043

go/vcs/discovery.go

@@ -59,6 +59,7 @@ func parseMetaGoImports(r io.Reader) (imports []metaImport, err error) {
 			})
 		}
 	}
+	return
 }
 
 // attrValue returns the attribute value for the case-insensitive key