document broker: handle tile request without tile cache

Change-Id: I5e0006cde07a84e6553db92627fdab943ac51d04
Reviewed-on: https://gerrit.libreoffice.org/c/online/+/103534
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>

wsd/DocumentBroker.cpp

@@ -1744,6 +1744,12 @@ void DocumentBroker::handleTileRequest(TileDesc& tile,
     const std::string tileMsg = tile.serialize();
     LOG_TRC("Tile request for " << tileMsg);
 
+    if (!hasTileCache())
+    {
+        LOG_WRN("Tile request without a loaded document?");
+        return;
+    }
+
     TileCache::Tile cachedTile = _tileCache->lookupTile(tile);
     if (cachedTile)
     {