Calibre plugin - Fixed hang when book has series but series index is nil (#8870)

plugins/calibre.koplugin/search.lua

@@ -271,7 +271,7 @@ function CalibreSearch:bookCatalog(t, option)
         local entry = {}
         entry.info = getBookInfo(book)
         entry.path = book.rootpath .. "/" .. book.lpath
-        if series then
+        if series and book.series_index then
             local major, minor = string.format("%05.2f", book.series_index):match("([^.]+).([^.]+)")
             if minor ~= "00" then
                 subseries = true