go.tools/cmd/vet: prevent panic on goto without label

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/107260043

cmd/vet/deadcode.go

@@ -81,7 +81,9 @@ func (d *deadState) findLabels(stmt ast.Stmt) {
 	case *ast.BranchStmt:
 		switch x.Tok {
 		case token.GOTO:
-			d.hasGoto[x.Label.Name] = true
+			if x.Label != nil {
+				d.hasGoto[x.Label.Name] = true
+			}
 
 		case token.BREAK:
 			stmt := d.breakTarget

cmd/vet/testdata/deadcode.go

@@ -2118,3 +2118,8 @@ var _ = func() int {
 	}
 	println() // ok
 }
+
+var _ = func() {
+	// goto without label used to panic
+	goto
+}