ES3: Fix crash when binding nullptr to DEPTH_STENCIL.
ES3: Fix crash when binding nullptr to DEPTH_STENCIL.

In some cases, binding nullptr could query for the internal format
of the null object, causing us to dereference null and crash.

BUG=angleproject:1080
TEST=dEQP-GLES3.functional.multisample.fbo_*

Change-Id: I592ab31dc1efadfac003c39051632fc2a0fff0cc
Reviewed-on: https://chromium-review.googlesource.com/286853
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Reviewed-by: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Tested-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

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