Apply image preview centering styles only if there's an image

For cases when we display a security warning about the image attachment
the image centering styles would make the warning centered, which
we do not want.

program/js/app.js

@@ -425,12 +425,19 @@ function rcube_webmail()
           }
 
           // center and scale the image in preview frame
+          // TODO: Find a better way. Onload is late, also we could use embed.css
           if (this.env.mimetype.startsWith('image/'))
             $(this.gui_objects.messagepartframe).on('load', function() {
-              var css = 'img { max-width:100%; max-height:100%; } ' // scale
-                + 'body { display:flex; align-items:center; justify-content:center; height:100%; margin:0; }'; // align
-
-              $(this).contents().find('head').append('<style type="text/css">'+ css + '</style>');
+              var contents = $(this).contents();
+
+              // do not apply styles to an error page (with no image)
+              if (contents.find('img').length)
+                contents.find('head').append(
+                  '<style type="text/css">'
+                  + 'img { max-width:100%; max-height:100%; } ' // scale
+                  + 'body { display:flex; align-items:center; justify-content:center; height:100%; margin:0; }' // align
+                  + '</style>'
+                );
             });
         }
         // show printing dialog