coLunacyDNS: Fix bug where we improperly use FD_ISSET w.r.t. select()

If you use FD_ISSET on a connection never set with FD_SET, this triggers
a buffer overflow in Ubuntu’s 20.04 glibc.  Fixed.

Found when updating tests for https://github.com/samboy/MaraDNS/issues/91

This *also* means we will need to make a coLunacyDNS 1.0.010

Author: Sam Trenholme

deadwood-github/tools/coLunacyDNS/coLunacyDNS.c

@@ -2065,7 +2065,8 @@ void runServer(lua_State *L) {
                         if(FD_ISSET(localConn4, &selectFdSet)) {
                                 sock4 = localConn4;
 				selectOut -= 1;
-			} else if(FD_ISSET(localConn6, &selectFdSet)) {
+			} else if(localConn6 != INVALID_SOCKET &&
+				  FD_ISSET(localConn6, &selectFdSet)) {
 				sock6 = localConn6;
 				selectOut -= 1;
                         } else {