Fix zero texture application.

Patch https://chromium-review.googlesource.com/#/c/227711/ would
allow NULL Textures sometimes from State::getSamplerTexture. Fix
this by always setting the zero "Default" textures instead of NULL
in the State.

This was breaking the ES2-CTS test 'framebuffer_objects' on Windows.

BUG=angle:826

Change-Id: Ie08a89cff0555f21c769759e0c0ed73456a2f91c
Reviewed-on: https://chromium-review.googlesource.com/228275
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>