cmd/go: don't crash in go fmt on invalid input when using modules

Fixes #26792

Change-Id: I9a878180af28e3939b654fd88bed150010dffde0
Reviewed-on: https://go-review.googlesource.com/127856
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/fmtcmd/fmt.go

@@ -60,7 +60,7 @@ func runFmt(cmd *base.Command, args []string) {
 		}()
 	}
 	for _, pkg := range load.PackagesAndErrors(args) {
-		if modload.Enabled() && !pkg.Module.Main {
+		if modload.Enabled() && pkg.Module != nil && !pkg.Module.Main {
 			if !printed {
 				fmt.Fprintf(os.Stderr, "go: not formatting packages in dependency modules\n")
 				printed = true

src/cmd/go/testdata/script/mod_gofmt_invalid.txt

+# Test for a crash in go fmt on invalid input when using modules.
+# Issue 26792.
+
+env GO111MODULE=on
+! go fmt x.go
+! stderr panic
+
+-- go.mod --
+module x
+
+-- x.go --
+// Missing package declaration.
+var V int