mesa/st: check buf before dereferencing it

It can be NULL.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36411>

src/mesa/state_tracker/st_draw_feedback.c

@@ -198,11 +198,12 @@ st_feedback_draw_vbo(struct gl_context *ctx,
       struct gl_buffer_binding *binding =
          &st->ctx->UniformBufferBindings[prog->sh.UniformBlocks[i]->Binding];
       struct gl_buffer_object *st_obj = binding->BufferObject;
-      struct pipe_resource *buf = st_obj->buffer;
 
-      if (!buf)
+      if (!st_obj || !st_obj->buffer)
          continue;
 
+      struct pipe_resource *buf = st_obj->buffer;
+
       unsigned offset = binding->Offset;
       unsigned size = buf->width0 - offset;