[continuelist addon] Fix list numbering error

Closes codemirror#5290

Author: ryu-sato

addon/edit/continuelist.js

@@ -66,7 +66,7 @@
         var newNumber = (parseInt(startItem[3], 10) + lookAhead - skipCount);
         var nextNumber = (parseInt(nextItem[3], 10)), itemNumber = nextNumber;
 
-        if (startIndent === nextIndent) {
+        if (startIndent === nextIndent && !isNaN(nextNumber)) {
           if (newNumber === nextNumber) itemNumber = nextNumber + 1;
           if (newNumber > nextNumber) itemNumber = newNumber + 1;
           cm.replaceRange(