remove incorrect out-of-memory code-path

The VBO setup code in gallium currently bails on rendering with a NULL resource in a buffer object, assuming there was an out-of-memory error during VBO setup. But this can also happen by using a VBO that was never assigned any data.

Both of these cases are well-defined in OpenGL, and means the same thing as a zero-size VBO. Rendering is perfectly well-defined for this (apart from the actual data coming out of such a VBO when the robustness bit is unset), so we shouldn't prevent rendering.