[FIX] mail: Allow to navigate quickly among the different attachments

What are the steps to reproduce your issue ?

    1. Create a record and add two attachments
    2. Navigate quickly between these attachments

What is currently happening ?

    Traceback
    TypeError: Cannot read property 'complete' of undefined
        at AttachmentViewer._handleImageLoad (https://www.odoo.com/mail/static/src/components/attachment_viewer/attachment_viewer.js:163:20

)

opw-2521901

closes odoo/odoo#70309

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/src/components/attachment_viewer/attachment_viewer.js

@@ -160,7 +160,7 @@ class AttachmentViewer extends Component {
         const image = refs[`image_${this.attachmentViewer.attachment.id}`];
         if (
             this.attachmentViewer.attachment.fileType === 'image' &&
-            !image.complete
+            (!image || !image.complete)
         ) {
             this.attachmentViewer.update({ isImageLoading: true });
         }