gallium: check ctx->DrawBuffer before flushing bitmap cache.

During context unbind, we may not have a draw buffer.
This fixes demos/tunnel2.c

src/mesa/state_tracker/st_cb_bitmap.c

@@ -547,6 +547,7 @@ void
 st_flush_bitmap_cache(struct st_context *st)
 {
    if (!st->bitmap.cache->empty) {
+      if (st->ctx->DrawBuffer) {
          struct bitmap_cache *cache = st->bitmap.cache;
          struct pipe_context *pipe = st->pipe;
          struct pipe_screen *screen = pipe->screen;
@@ -577,6 +578,7 @@ st_flush_bitmap_cache(struct st_context *st)
                           BITMAP_CACHE_WIDTH, BITMAP_CACHE_HEIGHT,
                           cache->texture);
 
+      }
       reset_cache(st);
    }
 }