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/294830Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>