llvmpipe: Ensure transfers are mapped.

This shouldn't happen but it does by some misterious reason. Fail the
assertion but at least do not segfault on release builds.

src/gallium/drivers/llvmpipe/lp_tile_cache.c

@@ -290,6 +290,10 @@ lp_get_cached_tile(struct llvmpipe_tile_cache *tc,
    
    assert(tc->surface);
    assert(tc->transfer);
+   assert(tc->transfer_map);
+
+   if(!tc->transfer_map)
+      lp_tile_cache_map_transfers(tc);
 
    switch(tile->status) {
    case LP_TILE_STATUS_CLEAR: