When doing a masked clear with only a depth-stencil buffer, we would
look for the zero-th element of an empty vector.

Change-Id: I17d61cf582c6e91fd46f8a8e13e35c6d32dfa109
Reviewed-on: https://chromium-review.googlesource.com/181554Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>