Fix mipmap generation on undefined cube texture

Fix mipmap generation on undefined cube texture

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>