st/mesa: empty buffer binding if the buffer's not really there

This can happen with 0-sized buffers.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/state_tracker/st_atom_atomicbuf.c

@@ -58,9 +58,11 @@ st_bind_atomics(struct st_context *st,
          st_buffer_object(binding->BufferObject);
       struct pipe_shader_buffer sb = { 0 };
 
+      if (st_obj && st_obj->buffer) {
          sb.buffer = st_obj->buffer;
          sb.buffer_offset = binding->Offset;
          sb.buffer_size = st_obj->buffer->width0 - binding->Offset;
+      }
 
       st->pipe->set_shader_buffers(st->pipe, shader_type,
                                    atomic->Binding, 1, &sb);