Fix possible use of garbage value in acquirePaintingResource

If ASSERT_NOT_REACHED is compiled out (like in release builds), then
hasFallback can be used uninitialized below.

Review URL: https://codereview.chromium.org/196113002

git-svn-id: svn://svn.chromium.org/blink/trunk@169000 bbb929c8-8fbe-4397-9dbb-9b2b20218538