Fix zero-size textures and unpack buffers. We would sometimes attempt to use a NULL staging texture when initializing a zero size texture when using unpack buffers. Change-Id: I2e03141c19d9c47eeffb0d56c03bcfe476a9e48d Reviewed-on: https://chromium-review.googlesource.com/181781 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Nicolas Capens <nicolascapens@chromium.org> Commit-Queue: Nicolas Capens <nicolascapens@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>