FramebufferAttachment: do not attach with an uninitialized mResource Otherwise attach will try to call onDetach on mResource, causing an UB (and a segfault in most cases). This fixes a crash when running angle_unittests on Mac with another patch. BUG=angleproject:891 Change-Id: I09d34d7246849059a13bc2fb00a61837cccd97d7 Reviewed-on: https://chromium-review.googlesource.com/294830 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>