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/181781Reviewed-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>