Fix crash: use util.startswith instead of string method

main.lua

@@ -596,7 +596,7 @@ function Bookhoard:setupOPDS()
         if server.url and (
             server.url == opds_url
             or server.url == opds_base_url
-            or server.url:startswith(opds_base_url .. "?")
+            or util.startswith(server.url, opds_base_url .. "?")
         ) then
             servers[i] = {
                 title = "Bookhoard",