core: restore data resource creation diffs

cd0c452 contained a bug where the creation diff for a data resource was
put into a new local variable within the else block rather than into the
diff variable in the parent scope, causing a null diff to always be
produced.

This restores the expected behavior: a computed data resource appears in
the diff, so it can then be fetched during the apply walk.

terraform/context_plan_test.go

@@ -889,10 +889,26 @@ func TestContext2Plan_computedDataResource(t *testing.T) {
 	if _, ok := moduleDiff.Resources["aws_instance.foo"]; !ok {
 		t.Fatalf("missing diff for aws_instance.foo")
 	}
-	_, ok := moduleDiff.Resources["data.aws_vpc.bar"]
+	iDiff, ok := moduleDiff.Resources["data.aws_vpc.bar"]
 	if !ok {
 		t.Fatalf("missing diff for data.aws_vpc.bar")
 	}
+
+	expectedDiff := &InstanceDiff{
+		Attributes: map[string]*ResourceAttrDiff{
+			"id": {
+				NewComputed: true,
+				RequiresNew: true,
+				Type:        DiffAttrOutput,
+			},
+		},
+	}
+	if same, _ := expectedDiff.Same(iDiff); !same {
+		t.Fatalf(
+			"incorrect diff for data.aws_vpc.bar\ngot:  %#v\nwant: %#v",
+			iDiff, expectedDiff,
+		)
+	}
 }
 
 func TestContext2Plan_computedList(t *testing.T) {

terraform/eval_read_data.go

@@ -38,7 +38,8 @@ func (n *EvalReadDataDiff) Eval(ctx EvalContext) (interface{}, error) {
 		provider := *n.Provider
 		config := *n.Config
 
-		diff, err := provider.ReadDataDiff(n.Info, config)
+		var err error
+		diff, err = provider.ReadDataDiff(n.Info, config)
 		if err != nil {
 			return nil, err
 		}