cmd/go: avoid panic on 'go mod' without arguments

Fixes #26738

Change-Id: Icede3f59acb5b0e388660653cefc24a195b5d43b
GitHub-Last-Rev: 9989b74c12e94163c1f18859485a4ac0d74c5453
GitHub-Pull-Request: golang/go#26739
Reviewed-on: https://go-review.googlesource.com/127160Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/main.go

@@ -193,6 +193,8 @@ BigCmdLoop:
 				args = args[1:]
 				if len(args) == 0 {
 					help.PrintUsage(os.Stderr, bigCmd)
+					base.SetExitStatus(2)
+					base.Exit()
 				}
 				if args[0] == "help" {
 					// Accept 'go mod help' and 'go mod help foo' for 'go help mod' and 'go help mod foo'.