x/vgo: fix incorrect go.mod panic

Checks f.Module first, if f.Module == nil then init it.

Fixes golang/go#24181

Change-Id: I4e8cfb688654e89422ae14562283188a483079ca
Reviewed-on: https://go-review.googlesource.com/106800
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

vendor/cmd/go/internal/vgo/init.go

diff --git a/vendor/cmd/go/internal/vgo/init.go b/vendor/cmd/go/internal/vgo/init.go
index bb6bd7b..b6e2e7c 100644
--- a/vendor/cmd/go/internal/vgo/init.go
+++ b/vendor/cmd/go/internal/vgo/init.go
@@ -127,7 +127,7 @@
 	}
 	modFile = f
 
-	if len(f.Syntax.Stmt) == 0 {
+	if len(f.Syntax.Stmt) == 0 || f.Module == nil {
 		// Empty mod file. Must add module path.
 		path, err := FindModulePath(ModRoot)
 		if err != nil {