coverity#1240270 Copy-paste error

regression from

commit 03fe839d6e76ed8b1dfb65093ab59a8904852ff6
Date:   Sat Sep 20 15:41:13 2014 +0200
    html export: rework image output to use HTML writer

Change-Id: If50d4e3cc2d19ea35d41e848cb5b9f92f356bc59
coverity#1241102 Uninitialized pointer field

Change-Id: I10580e92da47b97d8d496366833e79a89cbd9260
coverity#1241396 Uninitialized pointer field

Change-Id: Ib9c38ae857125036252ba27ebd03f428046724e0
coverity#1241427 Unchecked dynamic_cast

Change-Id: Ie011d0bcc504c6c17a525abb368e69009a3d2d2e