roots: don't access a shared variable after releasing a lock

Access the local copy of `m_root_crawl_map` instead.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees-roots.cc

@@ -1632,7 +1632,7 @@ BeesRoots::insert_new_crawl()
 	lock.unlock();
 
 	// Nothing to crawl?  Seems suspicious...
-	if (m_root_crawl_map.empty()) {
+	if (crawl_map_copy.empty()) {
 		BEESLOGINFO("crawl map is empty!");
 	}