st/mesa: generate GL_OUT_OF_MEMORY if we can't create the index buffer

st/mesa: generate GL_OUT_OF_MEMORY if we can't create the index buffer

Before, if we failed to allocate the index buffer we'd silently
return from st_draw_vbo() without drawing anything.  We should
raise GL_OUT_OF_MEMORY to give some indication that something went
wrong.

Note: This is a candidate for the stable branches.

Reviewed-by: Marek Olšák <maraeo@gmail.com>