Fix failure initting 2D Array textures.

Our code currently failed in retrieving a NULL Image internally,
when we relied on getting texture dimensions on an unitialized
texture.

BUG=angle:813

Change-Id: Iaf6791ee291e45daae9c80d6452c4d9154227fdb
Reviewed-on: https://chromium-review.googlesource.com/229350
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>