cmd/vet: fix bug where incorrect exit code was returned

Fixes golang/go#8792.

This is a simple change that fixes the issue. It may be desireable
to opt for a larger code change that makes this problem less likely
to be inadvertedly reintroduced in the future. For instance, a vetMain()
func can be used similar to gofmtMain(), or the os.Exit call can be
deferred.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/150850043

cmd/vet/main.go

@@ -245,7 +245,7 @@ func main() {
 		for _, name := range flag.Args() {
 			walkDir(name)
 		}
-		return
+		os.Exit(exitCode)
 	}
 	if !doPackage(".", flag.Args()) {
 		warnf("no files checked")