This was causing crashes in Direct3D 9 applications using the "NULL" FOURCC
used as a render target format for rendering to depth only.

Bug 25351344

Change-Id: I69c2e0869f859489321c26dba56b7a80a2ed2f28
Reviewed-on: https://swiftshader-review.googlesource.com/5000Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>