BMessage: fix passing messages by area

I missed the fact that the BDataIO based Unflatten didn't handle this.
Restore the code from the recently removed buffer-based unflatten, so
it's possible to pass a message by area again.

src/kits/app/Message.cpp

@@ -1311,6 +1311,14 @@ BMessage::Unflatten(BDataIO* stream)
 
 	what = fHeader->what;
 
+	if ((fHeader->flags & MESSAGE_FLAG_PASS_BY_AREA) != 0
+		&& fHeader->message_area >= 0) {
+		status_t result = _Reference();
+		if (result != B_OK) {
+			_InitHeader();
+			return result;
+		}
+	} else {
 		fHeader->message_area = -1;
 
 		if (fHeader->field_count > 0) {
@@ -1325,6 +1333,7 @@ BMessage::Unflatten(BDataIO* stream)
 			if (result != fieldsSize)
 				return result < 0 ? result : B_BAD_VALUE;
 		}
+	}
 
 	if (fHeader->data_size > 0) {
 		fData = (uint8*)malloc(fHeader->data_size);