Fix infinite loop when the input of binarySearch is NaN

`utils.ipStr2Num` returns `NaN` if the `stringifiedIp` is a simple string, i.e.) `a`, it leads to infinite loop.
This infinite loop will consume system resources and can be used to create a denial of service attack

index.ts

@@ -104,6 +104,9 @@ function lookup4(stringifiedIp:string): Promise<ipInfo|null> {
   var nextIp:number = utils.ipStr2Num("255.255.255.255");
   return readFile<indexFile>("index.json")
     .then(function(data){
+      // IP cannot be NaN
+      if (Object.is(ip, NaN))
+          throw "IP cannot be NaN"
       rootIndex = utils.binarySearch(data, ip, utils.identity)
       if (rootIndex == -1){
         // Ip is not in the database, return empty object