llvmpipe,draw,gallivm: Ensure we don't walk beyond the end of the shader variant...

llvmpipe,draw,gallivm: Ensure we don't walk beyond the end of the shader variant list.

u_simple_list.h uses a sentinel element, and not a NULL element. So
ensure list is not empty when reducing the list of shader variants.

Something I noticed while trying to free variants more aggressively.

Reviewed-by: Brian Paul <brianp@vmware.com>