Fix the build with strict GCC 4.8

Unreviewed build fix.

Initialize dragImage to suppress GCC's warning about it being possibly used uninitialized,
which results is a build failure with GCC 4.8 that sets -Werror=maybe-uninitialized.

No new tests needed.

* page/DragController.cpp:
(WebCore::DragController::doImageDrag):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147922 268f45cc-cd09-0410-ab3c-d52691b4dbfc