Fix crash on missing braces

Fixes crash reported here https://github.com/hashicorp/terraform/issues/4350

hcl/parser/parser.go

@@ -151,7 +151,7 @@ func (p *Parser) objectItem() (*ast.ObjectItem, error) {
 
 	// do a look-ahead for line comment
 	p.scan()
-	if o.Val.Pos().Line == keys[0].Pos().Line && p.lineComment != nil {
+	if len(keys) > 0 && o.Val.Pos().Line == keys[0].Pos().Line && p.lineComment != nil {
 		o.LineComment = p.lineComment
 		p.lineComment = nil
 	}

hcl/parser/parser_test.go

@@ -292,6 +292,10 @@ func TestParse(t *testing.T) {
 			"array_comment_2.hcl",
 			true,
 		},
+		{
+			"missing_braces.hcl",
+			true,
+		},
 	}
 
 	const fixtureDir = "./test-fixtures"

hcl/parser/test-fixtures/missing_braces.hcl

@@ -0,0 +1,4 @@
+# should error, but not crash
+resource "template_file" "cloud_config" {
+  template = "$file("${path.module}/some/path")"
+}