Cube textures must be cube complete to generate mipmaps, but when the
base level is undefined the glGenerateMipmap command must be silently
ignored. This was previously leading to a null dereference.

Bug chromium:924022
Bug https://gitlab.khronos.org/opengl/API/issues/72

Change-Id: I5d6e8533118e554efa12045fc376126c7b00f263
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27491
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>