Reduce CPU consumption with nsock engine poll

Entering nsock_loop() with the poll engine activated and no registered FD
(timers only, for instance) should not directly return, but sleep until next
timeout.

CHANGELOG

@@ -1,5 +1,9 @@
 # Nmap Changelog ($Id$); -*-text-*-
 
+o Reduce CPU consumption when using nsock poll engine with no registered FD,
+  by actually calling Poll() for the time until timeout, instead of directly
+  returning zero and entering the loop again. [Henri Doreau]
+
 o Fix ICMP Echo (-PE) host discovery for IPv6, broken since 6.45, caused by
   failing to set the ICMP ID for outgoing packets which is used to match
   incoming responses. [Andrew Waters]

nsock/src/engine_poll.c

@@ -347,11 +347,7 @@ int poll_loop(struct npool *nsp, int msec_timeout) {
 #endif
 #endif
     {
-      if (pinfo->max_fd > -1)
       results_left = Poll(pinfo->events, pinfo->max_fd + 1, combined_msecs);
-      else
-        results_left = 0;
-
       if (results_left == -1)
         sock_err = socket_errno();
     }