Check for invalid custom Upstream Server in DNS Interceptor

[neelanjan00]

BUG REPORT

What happened: DNS interceptor tool supports using a custom upstream server. However, it doesn't validate if the upstream server is valid or not. Also, the checkValidUpstream function doesn't check for nil output from net.ParseIP function, hence returning incorrect boolean output from the parent function.

How to reproduce it (as minimally and precisely as possible): Use the DNS interceptor tool with incorrect upstream server env