Sometime we have a clear call with clearDepth+clearStencil but no
attachment to clear. This was triggering the assert instead of
just skipping the clear.

Bug: angleproject:2597
Change-Id: I2d7750db919afe47cf02155f7c4afd0b02318bb1
Reviewed-on: https://chromium-review.googlesource.com/1106039
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>